ipv4: Change rt->rt_iif encoding.

On input packet processing, rt->rt_iif will be zero if we should
use skb->dev->ifindex.

Since we access rt->rt_iif consistently via inet_iif(), that is
the only spot whose interpretation have to adjust.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/route.h b/include/net/route.h
index 60d611d..c29ef27 100644
--- a/include/net/route.h
+++ b/include/net/route.h
@@ -277,7 +277,11 @@
 
 static inline int inet_iif(const struct sk_buff *skb)
 {
-	return skb_rtable(skb)->rt_iif;
+	int iif = skb_rtable(skb)->rt_iif;
+
+	if (iif)
+		return iif;
+	return skb->skb_iif;
 }
 
 extern int sysctl_ip_default_ttl;