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/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index f6aa751..9d8223c 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -1080,6 +1080,10 @@
 	p->is_seen = is_seen;
 }
 
+void retire_sysctl_set(struct ctl_table_set *set)
+{
+	WARN_ON(!list_empty(&set->list));
+}
 
 int __init proc_sys_init(void)
 {