io_uring: don't mark link's head for_async

No reason to mark a head of a link as for-async in io_req_defer_prep().
grab_env(), etc. That will be done further during submission if
neccessary.

Mark for_async=false saving extra grab_env() in many cases.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -6092,7 +6092,7 @@ static int io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 			if (io_alloc_async_ctx(req))
 				return -EAGAIN;
 
-			ret = io_req_defer_prep(req, sqe, true);
+			ret = io_req_defer_prep(req, sqe, false);
 			if (ret)
 				req->flags |= REQ_F_FAIL_LINK;
 			*link = req;