blk-throttle: use 'READ/WRITE' instead of '0/1'

Make the code easier to read, like everywhere else.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220827101637.1775111-2-yukuai1@huaweicloud.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.c

@@ -329,8 +329,8 @@ static struct bio *throtl_pop_queued(struct list_head *queued,
 /* init a service_queue, assumes the caller zeroed it */
 static void throtl_service_queue_init(struct throtl_service_queue *sq)
 {
-	INIT_LIST_HEAD(&sq->queued[0]);
-	INIT_LIST_HEAD(&sq->queued[1]);
+	INIT_LIST_HEAD(&sq->queued[READ]);
+	INIT_LIST_HEAD(&sq->queued[WRITE]);
 	sq->pending_tree = RB_ROOT_CACHED;
 	timer_setup(&sq->pending_timer, throtl_pending_timer_fn, 0);
 }
@@ -1151,7 +1151,7 @@ static int throtl_select_dispatch(struct throtl_service_queue *parent_sq)
 		nr_disp += throtl_dispatch_tg(tg);
 
 		sq = &tg->service_queue;
-		if (sq->nr_queued[0] || sq->nr_queued[1])
+		if (sq->nr_queued[READ] || sq->nr_queued[WRITE])
 			tg_update_disptime(tg);
 
 		if (nr_disp >= THROTL_QUANTUM)