btrfs/219: fix _cleanup() to successful release the loop-device

btrfs/219: fix _cleanup() to successful release the loop-device

When we fail with the message 'We were allowed to mount when we should
have failed,' it will fail to clean up the loop devices, making it
difficult to run further test cases or the same test case again.

So we need a 2nd loop device local variable to release it. Let's
reorganize the local variables to clean them up in the _cleanup() function.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>