[XFRM]: Add generation count to xfrm_state and xfrm_dst.

Each xfrm_state inserted gets a new generation counter
value.  When a bundle is created, the xfrm_dst objects
get the current generation counter of the xfrm_state
they will attach to at dst->xfrm.

xfrm_bundle_ok() will return false if it sees an
xfrm_dst with a generation count different from the
generation count of the xfrm_state that dst points to.

This provides a facility by which to passively and
cheaply invalidate cached IPSEC routes during SA
database changes.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/xfrm.h

@@ -104,6 +104,8 @@ struct xfrm_state
 	struct xfrm_id		id;
 	struct xfrm_selector	sel;
 
+	u32			genid;
+
 	/* Key manger bits */
 	struct {
 		u8		state;
@@ -590,6 +592,7 @@ struct xfrm_dst
 		struct rt6_info		rt6;
 	} u;
 	struct dst_entry *route;
+	u32 genid;
 	u32 route_mtu_cached;
 	u32 child_mtu_cached;
 	u32 route_cookie;