io_uring: don't io_prep_async_work() linked reqs

There is no real reason left for preparing io-wq work context for linked
requests in advance, remove it as this might become a bottleneck in some
cases.
Reported-by: Roman Gershman <romger@amazon.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5672,9 +5672,6 @@ static int io_req_defer_prep(struct io_kiocb *req,
 	ret = io_prep_work_files(req);
 	if (unlikely(ret))
 		return ret;
-
-	io_prep_async_work(req);
-
 	return io_req_prep(req, sqe);
 }