tcp_io: fix write_cb busy-loop with deferred callback — turn off write_monitor after flush when waiter_defer=1

lib/tcp_io.c

@@ -387,6 +387,10 @@ static void write_cb(socket_t sock, void* arg) {
     if (!tc->write_buf) {
         DEBUG_TRACE(DEBUG_CATEGORY_SOCKET, "tcp_io: write_cb flushed fd=%d wmon=%d (resume WQ cb)", (int)tc->sock, tc->write_monitor);
         queue_resume_callback(tc->write_queue);
+        if (tc->write_queue->waiter_defer && tc->write_monitor) {
+            uasync_set_socket_write(tc->ua, tc->socket_id, 0);
+            tc->write_monitor = 0;
+        }
     }
 }