virtio_net: flush when in xmit_more mode and under descriptor pressure

Mirror the changes made to ixgbe in commit 2367a17390138f68b3aa28f2f220b8d7ff8d91f4
("ixgbe: flush when in xmit_more mode and under descriptor pressure")

Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -953,7 +953,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 		}
 	}
 
-	if (!skb->xmit_more)
+	if (__netif_subqueue_stopped(dev, qnum) || !skb->xmit_more)
 		virtqueue_kick(sq->vq);
 
 	return NETDEV_TX_OK;