netns: Add an explicit rcu_barrier to unregister_pernet_{device|subsys}

This allows namespace exit methods to batch work that comes requires an
rcu barrier using call_rcu without having to treat the
unregister_pernet_operations cases specially.
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -413,8 +413,11 @@ static int register_pernet_operations(struct list_head *list,
 		}
 	}
 	error = __register_pernet_operations(list, ops);
-	if (error && ops->id)
-		ida_remove(&net_generic_ids, *ops->id);
+	if (error) {
+		rcu_barrier();
+		if (ops->id)
+			ida_remove(&net_generic_ids, *ops->id);
+	}
 
 	return error;
 }
@@ -423,6 +426,7 @@ static void unregister_pernet_operations(struct pernet_operations *ops)
 {
 	
 	__unregister_pernet_operations(ops);
+	rcu_barrier();
 	if (ops->id)
 		ida_remove(&net_generic_ids, *ops->id);
 }