[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/net/sched/em_meta.c b/net/sched/em_meta.c
index 3da4129..72cf86e 100644
--- a/net/sched/em_meta.c
+++ b/net/sched/em_meta.c
@@ -256,10 +256,10 @@
 
 META_COLLECTOR(int_rtiif)
 {
-	if (unlikely(skb->dst == NULL))
+	if (unlikely(skb->rtable == NULL))
 		*err = -1;
 	else
-		dst->value = ((struct rtable*) skb->dst)->fl.iif;
+		dst->value = skb->rtable->fl.iif;
 }
 
 /**************************************************************************