virtio_net: stricter short buffer length checks

Our buffer length check is not strict enough for mergeable
buffers: buffer can still be shorter that header + address
by 2 bytes.

Fix that up.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>

drivers/net/virtio_net.c

@@ -437,7 +437,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
 	struct sk_buff *skb;
 	struct virtio_net_hdr_mrg_rxbuf *hdr;
 
-	if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
+	if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
 		pr_debug("%s: short packet %i\n", dev->name, len);
 		dev->stats.rx_length_errors++;
 		if (vi->mergeable_rx_bufs) {