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

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: f67676d160 ("io_uring: ensure async punted read/write requests copy iovec")
Reported-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1786,6 +1786,9 @@ 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;