net: add _skb_dst opaque field

struct sk_buff uses one union to define dst and rtable fields.

We want to replace direct access to these pointers by accessors.

First patch adds a new "unsigned long _skb_dst;" opaque field
in this union.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -325,6 +325,7 @@ struct sk_buff {
 	union {
 		struct  dst_entry	*dst;
 		struct  rtable		*rtable;
+		unsigned long		_skb_dst;
 	};
 #ifdef CONFIG_XFRM
 	struct	sec_path	*sp;