xen/netfront: remove unnecessary wmb

RING_PUSH_REQUESTS_AND_CHECK_NOTIFY is already able to make sure backend sees
requests before req_prod is updated.
Signed-off-by: NJacob Wen <jian.w.wen@oracle.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
Reviewed-by: NWei Liu <wei.liu2@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -337,8 +337,6 @@ static void xennet_alloc_rx_buffers(struct netfront_queue *queue)
 		return;
 	}
 
-	wmb();		/* barrier so backend seens requests */
-
 	RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&queue->rx, notify);
 	if (notify)
 		notify_remote_via_irq(queue->rx_irq);