net: Make skb->skb_iif always track skb->dev

Make it follow device decapsulation, from things such as VLAN and
bonding.

The stuff that actually cares about pre-demuxed device pointers, is
handled by the "orig_dev" variable in __netif_receive_skb().  And
the only consumer of that is the po->origdev feature of AF_PACKET
sockets.

Signed-off-by: David S. Miller <davem@davemloft.net>