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 <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/loop.c

···
 static int lo_release(struct gendisk *disk, fmode_t mode)
 {
 	struct loop_device *lo = disk->private_data;
+	int err;
 
 	mutex_lock(&lo->lo_ctl_mutex);
 
···
 		 * In autoclear mode, stop the loop thread
 		 * and remove configuration after last close.
 		 */
-		loop_clr_fd(lo, NULL);
+		err = loop_clr_fd(lo, NULL);
+		if (!err)
+			goto out_unlocked;
 	} else {
 		/*
 		 * Otherwise keep thread (if running) and config,
···
 
 out:
 	mutex_unlock(&lo->lo_ctl_mutex);
-
+out_unlocked:
 	return 0;
 }