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: Eric W. Biederman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/net_namespace.c

···
 		}
 	}
 	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;
 }
···
 {
 	
 	__unregister_pernet_operations(ops);
+	rcu_barrier();
 	if (ops->id)
 		ida_remove(&net_generic_ids, *ops->id);
 }