io_uring: don't setup async context for read/write fixed

to #26323578

commit 74566df3a71c1b92da608868cca787557d8be7b2 upstream.

We don't need it, and if we have it, then the retry handler will attempt
to copy the non-existent iovec with the inline iovec, with a segment
count that doesn't make sense.

Fixes: f67676d160c6 ("io_uring: ensure async punted read/write requests copy iovec")
Reported-by: NJonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Acked-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

fs/io_uring.c

@@ -1782,6 +1782,9 @@ static int io_setup_async_rw(struct io_kiocb *req, ssize_t io_size,
 			     struct iovec *iovec, struct iovec *fast_iov,
 			     struct iov_iter *iter)
 {
+	if (req->opcode == IORING_OP_READ_FIXED ||
+	    req->opcode == IORING_OP_WRITE_FIXED)
+		return 0;
 	if (!req->io && io_alloc_async_ctx(req))
 		return -ENOMEM;