blk-throttle: don't check whether or not lower limit is valid if CONFIG_BLK_DEV_THROTTLING_LOW is off

blk_throtl_update_limit_valid() will search for descendants to see if
'LIMIT_LOW' of bps/iops and READ/WRITE is nonzero. However, they're always
zero if CONFIG_BLK_DEV_THROTTLING_LOW is not set, furthermore, a lot of
time will be wasted to iterate descendants.

Thus do nothing in blk_throtl_update_limit_valid() in such situation.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.c

@@ -587,6 +587,7 @@ static void throtl_pd_online(struct blkg_policy_data *pd)
 	tg_update_has_rules(tg);
 }
 
+#ifdef CONFIG_BLK_DEV_THROTTLING_LOW
 static void blk_throtl_update_limit_valid(struct throtl_data *td)
 {
 	struct cgroup_subsys_state *pos_css;
@@ -607,6 +608,11 @@ static void blk_throtl_update_limit_valid(struct throtl_data *td)
 
 	td->limit_valid[LIMIT_LOW] = low_valid;
 }
+#else
+static inline void blk_throtl_update_limit_valid(struct throtl_data *td)
+{
+}
+#endif
 
 static void throtl_upgrade_state(struct throtl_data *td);
 static void throtl_pd_offline(struct blkg_policy_data *pd)