sysctl: Implement retire_sysctl_set

This adds a small helper retire_sysctl_set to remove the intimate knowledge about
the how a sysctl_set is implemented from net/sysct_net.c

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 08cabbf..475ff0e 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -1072,6 +1072,7 @@
 extern void setup_sysctl_set(struct ctl_table_set *p,
 	struct ctl_table_set *parent,
 	int (*is_seen)(struct ctl_table_set *));
+extern void retire_sysctl_set(struct ctl_table_set *set);
 
 void register_sysctl_root(struct ctl_table_root *root);
 struct ctl_table_header *__register_sysctl_paths(