[NET]: Introduce union in struct dst_entry to hold 'next' pointer

This patch introduces an anonymous union to nicely express the fact that all
objects inherited from struct dst_entry should access to the generic 'next'
pointer but with appropriate type verification.

This patch is a prereq before following patches.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -37,7 +37,12 @@ struct sk_buff;
 
 struct dst_entry
 {
-	struct dst_entry        *next;
+	union {
+		struct dst_entry *next;
+		struct rtable    *rt_next;
+		struct rt6_info   *rt6_next;
+		struct dn_route  *dn_next;
+	};
 	atomic_t		__refcnt;	/* client references	*/
 	int			__use;
 	struct dst_entry	*child;