[IPV4]: Add 'rtable' field in struct sk_buff to alias 'dst' and avoid casts

(Anonymous) unions can help us to avoid ugly casts.

A common cast it the (struct rtable *)skb->dst one.

Defining an union like  :
union {
     struct dst_entry *dst;
     struct rtable *rtable;
};
permits to use skb->rtable in place.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
index 89cd011..8660cb0f 100644
--- a/include/net/inet_sock.h
+++ b/include/net/inet_sock.h
@@ -195,7 +195,7 @@
 
 static inline int inet_iif(const struct sk_buff *skb)
 {
-	return ((struct rtable *)skb->dst)->rt_iif;
+	return skb->rtable->rt_iif;
 }
 
 #endif	/* _INET_SOCK_H */