ipv4: Prepare for change of rt->rt_iif encoding.

Use inet_iif() consistently, and for TCP record the input interface of
cached RX dst in inet sock.

rt->rt_iif is going to be encoded differently, so that we can
legitimately cache input routes in the FIB info more aggressively.

When the input interface is "use SKB device index" the rt->rt_iif will
be set to zero.

This forces us to move the TCP RX dst cache installation into the ipv4
specific code, and as well it should since doing the route caching for
ipv6 is pointless at the moment since it is not inspected in the ipv6
input paths yet.

Also, remove the unlikely on dst->obsolete, all ipv4 dsts have
obsolete set to a non-zero value to force invocation of the check
callback.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sctp/protocol.c b/net/sctp/protocol.c
index 9c90811..1f89c4e 100644
--- a/net/sctp/protocol.c
+++ b/net/sctp/protocol.c
@@ -568,7 +568,7 @@
 /* What interface did this skb arrive on? */
 static int sctp_v4_skb_iif(const struct sk_buff *skb)
 {
-	return skb_rtable(skb)->rt_iif;
+	return inet_iif(skb);
 }
 
 /* Was this packet marked by Explicit Congestion Notification? */