[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/ipv4/ip_input.c b/net/ipv4/ip_input.c
index 65631391..d36e310 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -351,7 +351,7 @@
 	if (iph->ihl > 5 && ip_rcv_options(skb))
 		goto drop;
 
-	rt = (struct rtable*)skb->dst;
+	rt = skb->rtable;
 	if (rt->rt_type == RTN_MULTICAST)
 		IP_INC_STATS_BH(IPSTATS_MIB_INMCASTPKTS);
 	else if (rt->rt_type == RTN_BROADCAST)