nbd: remove bogus BUG_ON in NBD_CLEAR_QUE

The NBD_CLEAR_QUE ioctl has been deprecated for quite some time (its job
is now done by two other ioctls).  We should stop trying to make bogus
assertions in it.  Also, user-level code should remove calls to
NBD_CLEAR_QUE, ASAP.
Signed-off-by: NMichal Belczyk <belczyk@bsd.krakow.pl>
Signed-off-by: NPaul Clements <paul.clements@steeleye.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/block/nbd.c

@@ -751,7 +751,6 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 		 * This is for compatibility only.  The queue is always cleared
 		 * by NBD_DO_IT or NBD_CLEAR_SOCK.
 		 */
-		BUG_ON(!nbd->sock && !list_empty(&nbd->queue_head));
 		return 0;
 
 	case NBD_PRINT_DEBUG: