[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/udp_ipv4.c b/net/ipv4/udp_ipv4.c
index 40978de..fd14c2c 100644
--- a/net/ipv4/udp_ipv4.c
+++ b/net/ipv4/udp_ipv4.c
@@ -893,7 +893,7 @@
 	struct sock *sk;
 	struct udphdr *uh = udp_hdr(skb);
 	unsigned short ulen;
-	struct rtable *rt = (struct rtable*)skb->dst;
+	struct rtable *rt = skb->rtable;
 	__be32 saddr = ip_hdr(skb)->saddr;
 	__be32 daddr = ip_hdr(skb)->daddr;