xfrm: policy: remove garbage_collect callback

Just call xfrm_garbage_collect_deferred() directly.
This gets rid of a write to afinfo in register/unregister and allows to
constify afinfo later on.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index 6a7ff69..77ca91d 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -219,7 +219,7 @@ static inline int xfrm4_garbage_collect(struct dst_ops *ops)
 {
 	struct net *net = container_of(ops, struct net, xfrm.xfrm4_dst_ops);
 
-	xfrm4_policy_afinfo.garbage_collect(net);
+	xfrm_garbage_collect_deferred(net);
 	return (dst_entries_get_slow(ops) > ops->gc_thresh * 2);
 }