[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_options.c b/net/ipv4/ip_options.c
index baaedd9..df93a9c 100644
--- a/net/ipv4/ip_options.c
+++ b/net/ipv4/ip_options.c
@@ -107,7 +107,7 @@
 	sptr = skb_network_header(skb);
 	dptr = dopt->__data;
 
-	daddr = ((struct rtable*)skb->dst)->rt_spec_dst;
+	daddr = skb->rtable->rt_spec_dst;
 
 	if (sopt->rr) {
 		optlen  = sptr[sopt->rr+1];
@@ -258,7 +258,7 @@
 	unsigned char * optptr;
 	int optlen;
 	unsigned char * pp_ptr = NULL;
-	struct rtable *rt = skb ? (struct rtable*)skb->dst : NULL;
+	struct rtable *rt = skb ? skb->rtable : NULL;
 
 	if (!opt) {
 		opt = &(IPCB(skb)->opt);
@@ -558,7 +558,7 @@
 {
 	struct   ip_options * opt	= &(IPCB(skb)->opt);
 	unsigned char * optptr;
-	struct rtable *rt = (struct rtable*)skb->dst;
+	struct rtable *rt = skb->rtable;
 	unsigned char *raw = skb_network_header(skb);
 
 	if (opt->rr_needaddr) {
@@ -606,7 +606,7 @@
 	__be32 nexthop;
 	struct iphdr *iph = ip_hdr(skb);
 	unsigned char *optptr = skb_network_header(skb) + opt->srr;
-	struct rtable *rt = (struct rtable*)skb->dst;
+	struct rtable *rt = skb->rtable;
 	struct rtable *rt2;
 	int err;
 
@@ -631,13 +631,13 @@
 		}
 		memcpy(&nexthop, &optptr[srrptr-1], 4);
 
-		rt = (struct rtable*)skb->dst;
-		skb->dst = NULL;
+		rt = skb->rtable;
+		skb->rtable = NULL;
 		err = ip_route_input(skb, nexthop, iph->saddr, iph->tos, skb->dev);
-		rt2 = (struct rtable*)skb->dst;
+		rt2 = skb->rtable;
 		if (err || (rt2->rt_type != RTN_UNICAST && rt2->rt_type != RTN_LOCAL)) {
 			ip_rt_put(rt2);
-			skb->dst = &rt->u.dst;
+			skb->rtable = rt;
 			return -EINVAL;
 		}
 		ip_rt_put(rt);