[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/linux/skbuff.h b/include/linux/skbuff.h
index bbd8d00..7beb239 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -256,7 +256,10 @@
 	ktime_t			tstamp;
 	struct net_device	*dev;
 
-	struct  dst_entry	*dst;
+	union {
+		struct  dst_entry	*dst;
+		struct  rtable		*rtable;
+	};
 	struct	sec_path	*sp;
 
 	/*