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: Michal Belczyk <[e-mail]>
Signed-off-by: Paul Clements <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/block/nbd.c

···
 		 * 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: