btrfs/254: don't leave mount on test fs in case of failure/interruption

btrfs/254: don't leave mount on test fs in case of failure/interruption

If the test fails or is interrupted after mounting $scratch_dev3 inside
the test filesystem and before unmounting at test_add_device(), we leave
without being unable to unmount the test filesystem since it has a mount
inside it. This results in the need to manually unmount $scratch_dev3,
otherwise a subsequent run of fstests fails since the unmount of the
test device fails with -EBUSY.

Fix this by unmounting $scratch_dev3 ($seq_mnt) in the _cleanup()
function.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>