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>

net/core/dev.c

@@ -3173,8 +3173,6 @@ static int __netif_receive_skb(struct sk_buff *skb)
 	if (netpoll_receive_skb(skb))
 		return NET_RX_DROP;
 
-	if (!skb->skb_iif)
-		skb->skb_iif = skb->dev->ifindex;
 	orig_dev = skb->dev;
 
 	skb_reset_network_header(skb);
@@ -3186,6 +3184,7 @@ static int __netif_receive_skb(struct sk_buff *skb)
 	rcu_read_lock();
 
 another_round:
+	skb->skb_iif = skb->dev->ifindex;
 
 	__this_cpu_inc(softnet_data.processed);