fstests: btrfs verify hardening agaist duplicate fsid

fstests: btrfs verify hardening agaist duplicate fsid

We have a known bug in btrfs, that we let the device path be changed
after the device has been mounted. So using this loop hole the new
copied device would appears as if its mounted immediately after its
been copied. So this test case reproduces this issue.

For example:

Initially.. /dev/mmcblk0p4 is mounted as /

lsblk
NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
mmcblk0     179:0    0 29.2G  0 disk
|-mmcblk0p4 179:4    0    4G  0 part /
|-mmcblk0p2 179:2    0  500M  0 part /boot
|-mmcblk0p3 179:3    0  256M  0 part [SWAP]
`-mmcblk0p1 179:1    0  256M  0 part /boot/efi

btrfs fi show
Label: none  uuid: 07892354-ddaa-4443-90ea-f76a06accaba
    Total devices 1 FS bytes used 1.40GiB
    devid    1 size 4.00GiB used 3.00GiB path /dev/mmcblk0p4

Copy mmcblk0 to sda
dd if=/dev/mmcblk0 of=/dev/sda

And immediately after the copy completes the change in the device
superblock is notified which the automount scans using
btrfs device scan and the new device sda becomes the mounted root
device.

lsblk
NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda           8:0    1 14.9G  0 disk
|-sda4        8:4    1    4G  0 part /
|-sda2        8:2    1  500M  0 part
|-sda3        8:3    1  256M  0 part
`-sda1        8:1    1  256M  0 part
mmcblk0     179:0    0 29.2G  0 disk
|-mmcblk0p4 179:4    0    4G  0 part
|-mmcblk0p2 179:2    0  500M  0 part /boot
|-mmcblk0p3 179:3    0  256M  0 part [SWAP]
`-mmcblk0p1 179:1    0  256M  0 part /boot/efi
btrfs fi show /
Label: none  uuid: 07892354-ddaa-4443-90ea-f76a06accaba
    Total devices 1 FS bytes used 1.40GiB
    devid    1 size 4.00GiB used 3.00GiB path /dev/sda4

The bug is quite nasty that you can't either unmount /dev/sda4 or
/dev/mmcblk0p4. And the problem does not get solved until you take
the sda out of the system on to another system to change its fsid using
the 'btrfstune -u' command.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>