[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_gre.c b/net/ipv4/ip_gre.c
index e7821ba..f9ee844 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -619,7 +619,7 @@
 #ifdef CONFIG_NET_IPGRE_BROADCAST
 		if (ipv4_is_multicast(iph->daddr)) {
 			/* Looped back packet, drop it! */
-			if (((struct rtable*)skb->dst)->fl.iif == 0)
+			if (skb->rtable->fl.iif == 0)
 				goto drop;
 			tunnel->stat.multicast++;
 			skb->pkt_type = PACKET_BROADCAST;
@@ -699,7 +699,7 @@
 		}
 
 		if (skb->protocol == htons(ETH_P_IP)) {
-			rt = (struct rtable*)skb->dst;
+			rt = skb->rtable;
 			if ((dst = rt->rt_gateway) == 0)
 				goto tx_error_icmp;
 		}