net: Checks skb_dst to be NULL in inet_iif

In inet_iif check if skb_rtable is NULL for the skb and return
skb->skb_iif if it is.

This change allows inet_iif to be called before the dst
information has been set in the skb (e.g. when doing socket based
UDP GRO).

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/route.h

@@ -322,10 +322,11 @@ static inline struct rtable *ip_route_newports(struct flowi4 *fl4, struct rtable
 
 static inline int inet_iif(const struct sk_buff *skb)
 {
-	int iif = skb_rtable(skb)->rt_iif;
+	struct rtable *rt = skb_rtable(skb);
+
+	if (rt && rt->rt_iif)
+		return rt->rt_iif;
 
-	if (iif)
-		return iif;
 	return skb->skb_iif;
 }