io_uring: fix NULL mm in io_poll_task_func()

io_poll_task_func() hand-coded link submission forgetting to set
TASK_RUNNING, acquire mm, etc. Call existing helper for that,
i.e. __io_req_task_submit().
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4518,13 +4518,8 @@ static void io_poll_task_func(struct callback_head *cb)
 	struct io_kiocb *nxt = NULL;
 
 	io_poll_task_handler(req, &nxt);
-	if (nxt) {
-		struct io_ring_ctx *ctx = nxt->ctx;
-
-		mutex_lock(&ctx->uring_lock);
-		__io_queue_sqe(nxt, NULL, NULL);
-		mutex_unlock(&ctx->uring_lock);
-	}
+	if (nxt)
+		__io_req_task_submit(nxt);
 }
 
 static int io_poll_double_wake(struct wait_queue_entry *wait, unsigned mode,