virtio: fix tx_ stats in virtio_net

get_buf() gives the length written by the other side, which will be
zero.  We want to add the skb length.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

drivers/net/virtio_net.c

@@ -221,7 +221,7 @@ static void free_old_xmit_skbs(struct virtnet_info *vi)
 	while ((skb = vi->svq->vq_ops->get_buf(vi->svq, &len)) != NULL) {
 		pr_debug("Sent skb %p\n", skb);
 		__skb_unlink(skb, &vi->send);
-		vi->dev->stats.tx_bytes += len;
+		vi->dev->stats.tx_bytes += skb->len;
 		vi->dev->stats.tx_packets++;
 		kfree_skb(skb);
 	}