btrfs/027: unmount scratch device if test fails

btrfs/027: unmount scratch device if test fails

This test, btrfs/027, runs tests against different raid profiles in
a loop, if one of them aborts, it also fails the following ones with
errors like,

Test -m raid10 -d raid10
ERROR: /dev/xxx is mounted
Test -m raid5 -d raid5
ERROR: /dev/xxx is mounted
Test -m raid6 -d raid6
ERROR: /dev/xxx is mounted

_scratch_unmount is added to avoid the above.

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