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>
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index f0c2824..9359a13 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -953,7 +953,7 @@
 		}
 	}
 
-	if (!skb->xmit_more)
+	if (__netif_subqueue_stopped(dev, qnum) || !skb->xmit_more)
 		virtqueue_kick(sq->vq);
 
 	return NETDEV_TX_OK;