io_uring: optimise io_free_batch_list()

Delay reading the next node in io_free_batch_list(), allows the compiler
to load the value a bit later improving register spilling in some cases.
With gcc 11.1 it helped to move @task_refs variable from the stack to a
register and optimises out a couple of per request instructions.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/cc9fdfb6f72a4e8bc9918a5e9f2d97869a263ae4.1633373302.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2280,9 +2280,10 @@ static void io_free_batch_list(struct io_ring_ctx *ctx,
 		struct io_kiocb *req = container_of(node, struct io_kiocb,
 						    comp_list);
 
-		node = req->comp_list.next;
-		if (!req_ref_put_and_test(req))
+		if (!req_ref_put_and_test(req)) {
+			node = req->comp_list.next;
 			continue;
+		}
 
 		io_queue_next(req);
 		io_dismantle_req(req);
@@ -2294,6 +2295,7 @@ static void io_free_batch_list(struct io_ring_ctx *ctx,
 			task_refs = 0;
 		}
 		task_refs++;
+		node = req->comp_list.next;
 		wq_stack_add_head(&req->comp_list, &ctx->submit_state.free_list);
 	} while (node);