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>