user -> user_id for _remote_knock_client

synapse/handlers/room_member_worker.py

@@ -125,7 +125,7 @@ async def remote_knock(
         ret = await self._remote_knock_client(
             remote_room_hosts=remote_room_hosts,
             room_id=room_id,
-            user=user,
+            user_id=user.to_string,
             content=content,
         )
         return ret["event_id"], ret["stream_id"]