net/core: dev_forward_skb() should clear skb_iif

While investigating another bug, I found that the code on the incoming path
in __netif_receive_skb will only set skb->skb_iif if it is already 0.  When
dev_forward_skb() is used in the case of interfaces like veth, skb_iif may
already have been set.  Making dev_forward_skb() cause the packet to look
like a newly received packet would seem to the the correct behaviour here,
as otherwise the wrong incoming interface can be reported for such a packet.
Signed-off-by: NBenjamin LaHaise <bcrl@kvack.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1597,6 +1597,7 @@ int dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
 		kfree_skb(skb);
 		return NET_RX_DROP;
 	}
+	skb->skb_iif = 0;
 	skb_set_dev(skb, dev);
 	skb->tstamp.tv64 = 0;
 	skb->pkt_type = PACKET_HOST;