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 445409602c092
(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>
int dev_unicast_sync(struct net_device *to, struct net_device *from)
{
	int err = 0;

	if (to->addr_len != from->addr_len)
		return -EINVAL;

	netif_addr_lock_bh(to);
	err = __hw_addr_sync(&to->uc, &from->uc, to->addr_len);
	if (!err)
		__dev_set_rx_mode(to);
	netif_addr_unlock_bh(to);
	return err;
}

int dev_unicast_sync(struct net_device *to, struct net_device *from)
{
	int err = 0;

	if (to->addr_len != from->addr_len)
		return -EINVAL;

	netif_addr_lock_bh(to);
	err = __hw_addr_sync(&to->uc, &from->uc, to->addr_len);
	if (!err)
		__dev_set_rx_mode(to);
	netif_addr_unlock_bh(to);
	return err;
}
