[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/ipmr.c b/net/ipv4/ipmr.c
index a94f52c..7d63d74 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -1283,7 +1283,7 @@
 	if (vif_table[vif].dev != skb->dev) {
 		int true_vifi;
 
-		if (((struct rtable*)skb->dst)->fl.iif == 0) {
+		if (skb->rtable->fl.iif == 0) {
 			/* It is our own packet, looped back.
 			   Very complicated situation...
 
@@ -1357,7 +1357,7 @@
 int ip_mr_input(struct sk_buff *skb)
 {
 	struct mfc_cache *cache;
-	int local = ((struct rtable*)skb->dst)->rt_flags&RTCF_LOCAL;
+	int local = skb->rtable->rt_flags&RTCF_LOCAL;
 
 	/* Packet is looped back after forward, it should not be
 	   forwarded second time, but still can be delivered locally.
@@ -1594,7 +1594,7 @@
 {
 	int err;
 	struct mfc_cache *cache;
-	struct rtable *rt = (struct rtable*)skb->dst;
+	struct rtable *rt = skb->rtable;
 
 	read_lock(&mrt_lock);
 	cache = ipmr_cache_find(rt->rt_src, rt->rt_dst);