xen-netfront: be more drop monitor friendly

xennet_start_xmit() might copy skb with inappropriate layout
into a fresh one.

Old skb is freed, and at this point it is not a drop, but
a consume. New skb will then be either consumed or dropped.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -611,7 +611,7 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		nskb = skb_copy(skb, GFP_ATOMIC);
 		if (!nskb)
 			goto drop;
-		dev_kfree_skb_any(skb);
+		dev_consume_skb_any(skb);
 		skb = nskb;
 		page = virt_to_page(skb->data);
 		offset = offset_in_page(skb->data);