io_uring: fix incorrect io_kiocb reference in io_link_skb

In io_link_skb function, there is a bug where prev_notif is incorrectly
assigned using 'nd' instead of 'prev_nd'. This causes the context
validation check to compare the current notification with itself instead
of comparing it with the previous notification.

Fix by using the correct prev_nd parameter when obtaining prev_notif.

Signed-off-by: Yang Xiuwei <yangxiuwei@kylinos.cn>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Fixes: 6fe4220 ("io_uring/notif: implement notification stacking")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Yang Xiuwei

io_uring/notif.c

@@ -85,7 +85,7 @@ static int io_link_skb(struct sk_buff *skb, struct ubuf_info *uarg)
 		return -EEXIST;
 
 	prev_nd = container_of(prev_uarg, struct io_notif_data, uarg);
-	prev_notif = cmd_to_io_kiocb(nd);
+	prev_notif = cmd_to_io_kiocb(prev_nd);
 
 	/* make sure all noifications can be finished in the same task_work */
 	if (unlikely(notif->ctx != prev_notif->ctx ||