btrfs/011: umount device in _cleanup

btrfs/011: umount device in _cleanup

btrfs/011 replaces the first device in SCRATCH_DEV_POOL in test,
which is SCRATCH_DEV, and it fails to umount SCRATCH_MNT when it
aborts, because _cleanup doesn't do any umount. This may leave
SCRATCH_DEV not used but other devices from SCRATCH_DEV_POOL still
mounted on SCRATCH_MNT. Then this results in SCRATCH_DEV unusable
for subsequent tests because _require_scratch couldn't umount detect
& umount SCRATCH_DEV correctly.

Fix it by umounting SCRATCH_MNT in _cleanup, so the test always
umounts the devices on exit.

[eguan: add comments and rewrite summary and commit log]

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Tested-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>