io_uring: simplify drain ret passing

"ret" in io_drain_req() is only used in one place, remove it and pass
-ENOMEM directly.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ece724b77e66e6caabcc215e0032ee7ff140f289.1746788718.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -1672,7 +1672,6 @@ static __cold void io_drain_req(struct io_kiocb *req)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	struct io_defer_entry *de;
-	int ret;
 	u32 seq = io_get_sequence(req);
 
 	/* Still need defer if there is pending req in defer list. */
@@ -1689,8 +1688,7 @@ queue:
 	io_prep_async_link(req);
 	de = kmalloc(sizeof(*de), GFP_KERNEL_ACCOUNT);
 	if (!de) {
-		ret = -ENOMEM;
-		io_req_defer_failed(req, ret);
+		io_req_defer_failed(req, -ENOMEM);
 		return;
 	}