io_uring: clean up timeout async_data allocation

opcode prep functions are one of the first things that are called, we
can't have ->async_data allocated at this point and it's certainly a
bug. Reflect this assumption in io_timeout_prep() and add a WARN_ONCE
just in case.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/75a28ca7dbcc5af8b6cd9092819e8384c24dedd4.1634987320.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -6113,7 +6113,9 @@ static int io_timeout_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 	if (unlikely(off && !req->ctx->off_timeout_used))
 		req->ctx->off_timeout_used = true;
 
-	if (!req_has_async_data(req) && io_alloc_async_data(req))
+	if (WARN_ON_ONCE(req_has_async_data(req)))
+		return -EFAULT;
+	if (io_alloc_async_data(req))
 		return -ENOMEM;
 
 	data = req->async_data;