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: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: David S. Miller <davem@davemloft.net>