'df /dev/sda1' (rootfs) returns result for /dev on gce minimal images

[Robert C Jennings]

On base images the root FS shows up as /dev/sda1 in mount and df output. For minimized images the root FS shows up in df output as /dev/root. Scripts that call 'df /dev/sda1' to get information about the root FS will instead match to /dev not /

[Robert C Jennings (rcj)]

Launching GCE minimal bionic image from --image-family ubuntu-minimal-1804-lts --image-project ubuntu-os-cloud-devel

## This is wrong
$ df -h /dev/sda1
Filesystem Size Used Avail Use% Mounted on
devtmpfs 1.9G 0 1.9G 0% /dev

## Odd to see /dev/root not /dev/sda1
$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/root 3.0T 799M 3.0T 1% /
devtmpfs 1.9G 0 1.9G 0% /dev
<snip>

## Mount output shows /dev/sda1 not /dev/root
$ mount
/dev/sda1 on / type ext4 (rw,relatime,data=ordered)
devtmpfs on /dev type devtmpfs (rw,relatime,size=1887700k,nr_inodes=471925,mode=755)
/dev/sda15 on /boot/efi type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
<snip>

$ dmesg|grep root=
[ 0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.15.0-1008-gcp root=KPARTUUID=09fc90e1-2f42-4dfd-baf7-859f8a494247 ro scsi_mod.use_blk_mq=Y console=ttyS0
[ 0.000000] Kernel command line: BOOT_IMAGE=/boot/vmlinuz-4.15.0-1008-gcp root=KPARTUUID=09fc90e1-2f42-4dfd-baf7-859f8a494247 ro scsi_mod.use_blk_mq=Y console=ttyS0

## Both /dev/root and /dev/sda1 are the same device
$ ls -ln /dev|grep "8,"
crw------- 1 0 0 10 8, 0 Jun 5 22:30 ppp
brw------- 1 0 0 8, 1 Jun 5 22:30 root
brw-rw---- 1 0 6 8, 0 Jun 5 22:30 sda
brw-rw---- 1 0 6 8, 1 Jun 5 22:30 sda1
brw-rw---- 1 0 6 8, 14 Jun 5 22:30 sda14
brw-rw---- 1 0 6 8, 15 Jun 5 22:30 sda15

## df by mountpoint works
$ df -h /
Filesystem Size Used Avail Use% Mounted on
/dev/root 3.0T 799M 3.0T 1% /

## This is different from the non-minimzed image which would show /dev/sda1 instead
$ df -h /dev/root
Filesystem Size Used Avail Use% Mounted on
/dev/root 3.0T 799M 3.0T 1% /

$ df -h /dev/sda1
Filesystem Size Used Avail Use% Mounted on
devtmpfs 1.9G 0 1.9G 0% /dev