veth: Dont kfree_skb() after dev_forward_skb()

In case of congestion, netif_rx() frees the skb, so we must assume
dev_forward_skb() also consume skb.

Bug introduced by commit 4454096
(veth: move loopback logic to common location)

We must change dev_forward_skb() to always consume skb, and veth to not
double free it.

Bug report : http://marc.info/?l=linux-netdev&m=127310770900442&w=3

Reported-by: Martín Ferrari <martin.ferrari@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/veth.c

@@ -187,7 +187,6 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 	return NETDEV_TX_OK;
 
 rx_drop:
-	kfree_skb(skb);
 	rcv_stats->rx_dropped++;
 	return NETDEV_TX_OK;
 }

net/core/dev.c

@@ -1451,7 +1451,7 @@ static inline void net_timestamp(struct sk_buff *skb)
  *
  * return values:
  *	NET_RX_SUCCESS	(no congestion)
- *	NET_RX_DROP     (packet was dropped)
+ *	NET_RX_DROP     (packet was dropped, but freed)
  *
  * dev_forward_skb can be used for injecting an skb from the
  * start_xmit function of one device into the receive queue
@@ -1465,12 +1465,11 @@ int dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
 {
 	skb_orphan(skb);
 
-	if (!(dev->flags & IFF_UP))
-		return NET_RX_DROP;
-
-	if (skb->len > (dev->mtu + dev->hard_header_len))
+	if (!(dev->flags & IFF_UP) ||
+	    (skb->len > (dev->mtu + dev->hard_header_len))) {
+		kfree_skb(skb);
 		return NET_RX_DROP;
-
+	}
 	skb_set_dev(skb, dev);
 	skb->tstamp.tv64 = 0;
 	skb->pkt_type = PACKET_HOST;