[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/ipip.c b/net/ipv4/ipip.c
index dbaed69..894bce9 100644
--- a/net/ipv4/ipip.c
+++ b/net/ipv4/ipip.c
@@ -528,7 +528,7 @@
 
 	if (!dst) {
 		/* NBMA tunnel */
-		if ((rt = (struct rtable*)skb->dst) == NULL) {
+		if ((rt = skb->rtable) == NULL) {
 			tunnel->stat.tx_fifo_errors++;
 			goto tx_error;
 		}