ipv4: Properly purge netdev references on uncached routes.

When a device is unregistered, we have to purge all of the
references to it that may exist in the entire system.

If a route is uncached, we currently have no way of accomplishing
this.

So create a global list that is scanned when a network device goes
down.  This mirrors the logic in net/core/dst.c's dst_ifdown().

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index c628184..681ea2f 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -92,6 +92,7 @@
 	xdst->u.rt.rt_type = rt->rt_type;
 	xdst->u.rt.rt_gateway = rt->rt_gateway;
 	xdst->u.rt.rt_pmtu = rt->rt_pmtu;
+	INIT_LIST_HEAD(&xdst->u.rt.rt_uncached);
 
 	return 0;
 }