net:msg_zerocopy.sh in ubuntu_kernel_selftests flaky on Azure 4.15 (./msg_zerocopy: send: Invalid argument)

[Po-Hsu Lin]

This is not a regression, it's just that this issue was hinted without a bug report.

This issue can be found on instance Standard_B1ms only with:
  * X-Azure-4.15.0-1162.177~16.04.1
  * B-Azure-4.15.0-1162.177
  * B-Azure-4.15.0-1164.179

Test failed with:
 ./msg_zerocopy: send: Connection reset by peer

Test log:
 Running 'make run_tests -C net TEST_PROGS=msg_zerocopy.sh TEST_GEN_PROGS='' TEST_CUSTOM_PROGS='''
 make: Entering directory '/home/azure/autotest/client/tmp/ubuntu_kernel_selftests/src/linux/tools/testing/selftests/net'
 make --no-builtin-rules ARCH=x86 -C ../../../.. headers_install
 make[1]: Entering directory '/home/azure/autotest/client/tmp/ubuntu_kernel_selftests/src/linux'
   CHK include/generated/uapi/linux/version.h
 make[1]: Leaving directory '/home/azure/autotest/client/tmp/ubuntu_kernel_selftests/src/linux'
 TAP version 13
 selftests: msg_zerocopy.sh
 ========================================
 ipv4 tcp -t 1
 cpu: unable to pin, may increase variance.
 cpu: unable to pin, may increase variance.
 rx=36717 (4582 MB)
 ./msg_zerocopy: send: Connection reset by peer
 not ok 1..1 selftests: msg_zerocopy.sh [FAIL]

[Po-Hsu Lin (cypressyew)]

On ARM64 baremetal node blanka with N-nvidia-lowlatency 6.8.0-1015.16, this test was found flaky with "./msg_zerocopy: send: Connection refused":

 Running 'make run_tests -C net TEST_PROGS=msg_zerocopy.sh TEST_GEN_PROGS='' TEST_CUSTOM_PROGS='''
 make: Entering directory '/home/ubuntu/autotest/client/tmp/ubuntu_kselftests_net/src/linux/tools/testing/selftests/net'
 TAP version 13
 1..1
 # timeout set to 0
 # selftests: net: msg_zerocopy.sh
 # ipv4 tcp -t 1
 # tx=83811 (5230 MB) txc=0 zc=n
 # rx=41833 (5230 MB)
 # ipv4 tcp -z -t 1
 # tx=68309 (4262 MB) txc=68309 zc=n
 # rx=34118 (4262 MB)
 # ok
 # ipv6 tcp -t 1
 # tx=84907 (5298 MB) txc=0 zc=n
 # rx=42382 (5298 MB)
 # ipv6 tcp -z -t 1
 # tx=66429 (4145 MB) txc=66429 zc=n
 # rx=33193 (4145 MB)
 # ok
 # ipv4 udp -t 1
 # rx=125765 (7848 MB)
 # ./msg_zerocopy: send: Connection refused
 not ok 1 selftests: net: msg_zerocopy.sh # exit=1

Test passed after retest.