Fix node monitoring slot accounting for stop_watching()

NodeMonitorService::RemoveUserListeners() didn't decrement
io_context::num_monitors when removing a listener, so limit checks
would be off afterwards.

src/system/kernel/fs/node_monitor.cpp

@@ -982,6 +982,7 @@ NodeMonitorService::RemoveUserListeners(struct io_context *context,
 			// to remove its successor (which is saved in "removeListener")
 
 		_RemoveListener(removeListener);
+		context->num_monitors--;
 		count++;
 	}