loop: mutex already unlocked in loop_clr_fd()

mount/1865 is trying to release lock (&lo->lo_ctl_mutex) at:
but there are no more locks to release!

mutex is already unlocked in loop_clr_fd(), we should not
try to unlock it in lo_release() again.

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>