block/nbd: micro-optimization in nbd request completion

Add in-flight cmds to the tail. That way while searching
(during request completion),we will always get a hit on the
first element.

Signed-off-by: Chetan Loke <loke.chetan@gmail.com>
Acked-by: Paul.Clements@steeleye.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -481,7 +481,7 @@ static void nbd_handle_req(struct nbd_device *nbd, struct request *req)
 		nbd_end_request(req);
 	} else {
 		spin_lock(&nbd->queue_lock);
-		list_add(&req->queuelist, &nbd->queue_head);
+		list_add_tail(&req->queuelist, &nbd->queue_head);
 		spin_unlock(&nbd->queue_lock);
 	}