linux.h appears in
modules/drbd/drbd/drbd_fs.c
modules/drbd/drbd/drbd_main.c
modules/drbd/drbd/drbd_proc.c
modules/drbd/drbd/drbd_receiver.c
modules/drbd/drbd/drbd_req.c
modules/drbd/drbd/drbd_worker.c

However once that hurdle is passed, the next one appears
/usr/src/modules/drbd/drbd/drbd_main.c:52:35: error: linux/devfs_fs_kernel.h: No such file or directory

Which has also been removed (since 2.6.17), it only appears in modules/drbd/drbd/drbd_main.c so I removed the line. 

Next issue
if (drbd_ee_cache && kmem_cache_destroy(drbd_ee_cache))
                printk(KERN_ERR DEVICE_NAME
                       ": kmem_cache_destroy(drbd_ee_cache) FAILED\n");
Results in
/usr/src/modules/drbd/drbd/drbd_main.c:2023: error: void value not ignored as it ought to be

Also mentions that kmem_cache_t is deprecated

gcc (GCC) 4.1.2 20060928 (prerelease) (Ubuntu 4.1.1-13ubuntu5)wait...
Copyright (C) 2006 Free Software Foundation, Inc. build KVERS=2.6.20.3-custom-070319 KSRC=/usr/src/linux-headers-2.6.20.3-custom-070319 KDREV=2.6.20.3-customThis is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

GNU Make 3.81
Copyright (C) 2006  Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.

This program built for i486-pc-linux-gnu

dpatch  deapply-all
rm -rf patch-stamp patch-stampT debian/patched
dh_clean
/usr/bin/make -C drbd clean
make[1]: Entering directory `/usr/src/modules/drbd/drbd'
rm -rf .tmp_versions
rm -f *.[oas] *.ko .*.cmd .*.d .*.tmp *.mod.c .*.flags .depend .kernel*
make[1]: Leaving directory `/usr/src/modules/drbd/drbd'
/usr/bin/make  -f debian/rules kdist_clean kdist_config binary-modules
make[1]: Entering directory `/usr/src/modules/drbd'
dpatch  deapply-all
rm -rf patch-stamp patch-stampT debian/patched
dh_clean
/usr/bin/make -C drbd clean
make[2]: Entering directory `/usr/src/modules/drbd/drbd'
rm -rf .tmp_versions
rm -f *.[oas] *.ko .*.cmd .*.d .*.tmp *.mod.c .*.flags .depend .kernel*
make[2]: Leaving directory `/usr/src/modules/drbd/drbd'
/usr/bin/gcc-4.1
for templ in /usr/src/modules/drbd/debian/drbd8-module-_KVERS_.postinst /usr/src/modules/drbd/debian/drbd8-module-_KVERS_.postinst.backup /usr/src/modules/dr
bd/debian/drbd8-module-_KVERS_.postinst.modules.in; do \
    cp $templ `echo $templ | sed -e 's/_KVERS_/2.6.20.3-custom-070319/g'` ; \
  done
for templ in `ls debian/*.modules.in` ; do \
    test -e ${templ%.modules.in}.backup || cp ${templ%.modules.in} ${templ%.modules.in}.backup 2>/dev/null || true; \
    sed -e 's/##KVERS##/2.6.20.3-custom-070319/g ;s/#KVERS#/2.6.20.3-custom-070319/g ; s/_KVERS_/2.6.20.3-custom-070319/g ; s/##KDREV##/2.6.20.3-custom-07031
9-10.00.Custom/g ; s/#KDREV#/2.6.20.3-custom-070319-10.00.Custom/g ; s/_KDREV_/2.6.20.3-custom-070319-10.00.Custom/g  ' < $templ > ${templ%.modules.in}; \
  done
dh_testdir
dh_testroot
dh_clean -k
/usr/bin/make -C drbd KERNEL_SOURCES=/usr/src/linux-headers-2.6.20.3-custom-070319 MODVERSIONS=detect KERNEL=linux-2.6.20.3-custom-070319 KDIR=/usr/src/linux
-headers-2.6.20.3-custom-070319
make[2]: Entering directory `/usr/src/modules/drbd/drbd'

    Calling toplevel makefile of kernel source tree, which I believe is in
    KDIR=/usr/src/linux-headers-2.6.20.3-custom-070319

/usr/bin/make -C /usr/src/linux-headers-2.6.20.3-custom-070319 SUBDIRS=/usr/src/modules/drbd/drbd  modules
make[3]: Entering directory `/usr/src/linux-headers-2.6.20.3-custom-070319'
  CC [M]  /usr/src/modules/drbd/drbd/drbd_sizeof_sanity_check.o
  CC [M]  /usr/src/modules/drbd/drbd/drbd_buildtag.o
  CC [M]  /usr/src/modules/drbd/drbd/drbd_bitmap.o
In file included from /usr/src/modules/drbd/drbd/drbd_bitmap.c:34:
/usr/src/modules/drbd/drbd/drbd_int.h:1064: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_int.h:1065: warning: âkmem_cache_tâ is deprecated
  CC [M]  /usr/src/modules/drbd/drbd/drbd_fs.o
In file included from /usr/src/modules/drbd/drbd/drbd_fs.c:43:
/usr/src/modules/drbd/drbd/drbd_int.h:1064: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_int.h:1065: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_fs.c:396:2: warning: #warning checks below no longer valid
  CC [M]  /usr/src/modules/drbd/drbd/drbd_proc.o
In file included from /usr/src/modules/drbd/drbd/drbd_proc.c:40:
/usr/src/modules/drbd/drbd/drbd_int.h:1064: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_int.h:1065: warning: âkmem_cache_tâ is deprecated

  CC [M]  /usr/src/modules/drbd/drbd/drbd_worker.o
In file included from /usr/src/modules/drbd/drbd/drbd_worker.c:42:
/usr/src/modules/drbd/drbd/drbd_int.h:1064: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_int.h:1065: warning: âkmem_cache_tâ is deprecated
  CC [M]  /usr/src/modules/drbd/drbd/drbd_receiver.o
In file included from /usr/src/modules/drbd/drbd/drbd_receiver.c:53:
/usr/src/modules/drbd/drbd/drbd_int.h:1064: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_int.h:1065: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_receiver.c: In function âwarn_if_differ_considerablyâ:
/usr/src/modules/drbd/drbd/drbd_receiver.c:1835: warning: ISO C90 forbids mixed declarations and code
/usr/src/modules/drbd/drbd/drbd_receiver.c: In function âdrbd_do_authâ:
/usr/src/modules/drbd/drbd/drbd_receiver.c:2637: warning: implicit declaration of function âcrypto_hmacâ
/usr/src/modules/drbd/drbd/drbd_receiver.c: At top level:
/usr/src/modules/drbd/drbd/drbd_receiver.c:1578: warning: âdrbd_uuid_dumpâ defined but not used
  CC [M]  /usr/src/modules/drbd/drbd/drbd_req.o
In file included from /usr/src/modules/drbd/drbd/drbd_req.c:35:
/usr/src/modules/drbd/drbd/drbd_int.h:1064: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_int.h:1065: warning: âkmem_cache_tâ is deprecated
  CC [M]  /usr/src/modules/drbd/drbd/drbd_actlog.o
In file included from /usr/src/modules/drbd/drbd/drbd_actlog.c:30:
/usr/src/modules/drbd/drbd/drbd_int.h:1064: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_int.h:1065: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_actlog.c:321:2: warning: #warning check outcome of addition u64/sector_t/s32
  CC [M]  /usr/src/modules/drbd/drbd/lru_cache.o
  CC [M]  /usr/src/modules/drbd/drbd/drbd_main.o
In file included from /usr/src/modules/drbd/drbd/drbd_main.c:61:
/usr/src/modules/drbd/drbd/drbd_int.h:1064: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_int.h:1065: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_main.c:140: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_main.c:141: warning: âkmem_cache_tâ is deprecated
/usr/src/modules/drbd/drbd/drbd_main.c: In function â_drbd_set_stateâ:
/usr/src/modules/drbd/drbd/drbd_main.c:740: warning: ISO C90 forbids mixed declarations and code
/usr/src/modules/drbd/drbd/drbd_main.c: In function âdrbd_destroy_mempoolsâ:
/usr/src/modules/drbd/drbd/drbd_main.c:2024: error: void value not ignored as it ought to be
/usr/src/modules/drbd/drbd/drbd_main.c:2028: error: void value not ignored as it ought to be
/usr/src/modules/drbd/drbd/drbd_main.c: In function âdrbd_cleanupâ:
/usr/src/modules/drbd/drbd/drbd_main.c:2226: warning: implicit declaration of function âdevfs_removeâ
/usr/src/modules/drbd/drbd/drbd_main.c: In function âdrbd_initâ:
/usr/src/modules/drbd/drbd/drbd_main.c:2319: warning: implicit declaration of function âdevfs_mk_dirâ
/usr/src/modules/drbd/drbd/drbd_main.c:2343: error: âstruct gendiskâ has no member named âdevfs_nameâ
make[4]: *** [/usr/src/modules/drbd/drbd/drbd_main.o] Error 1
make[3]: *** [_module_/usr/src/modules/drbd/drbd] Error 2
make[3]: Leaving directory `/usr/src/linux-headers-2.6.20.3-custom-070319'
make[2]: *** [kbuild] Error 2
make[2]: Leaving directory `/usr/src/modules/drbd/drbd'
make[1]: *** [binary-modules] Error 2
make[1]: Leaving directory `/usr/src/modules/drbd'
make: *** [kdist_build] Error 2