block/blk-cgroup-rwstat: use data_race() for online test

blkg_rwstat_recursive_sum() reads pos_blkg->online without the queue
lock that its doc comment requires, since blkcg_print_blkgs() stopped
holding it in 56cc24f59c.  Concurrent blkg_create/destroy flips
->online, tripping KCSAN.  The race is harmless (RCU-protected, stale
online only causes minor stat noise).  Use data_race() to annotate
the intentional lockless read.

Also update the stale doc comment that still requires the queue lock.

Reviewed-by: Yu Kuai <yukuai@fygo.io>
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://patch.msgid.link/20260803133400.137906-1-cui.tao@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup-rwstat.c

@@ -88,8 +88,7 @@ EXPORT_SYMBOL_GPL(blkg_prfill_rwstat);
  * @sum: blkg_rwstat_sample structure containing the results
  *
  * Collect the blkg_rwstat specified by @blkg, @pol and @off and all its
- * online descendants and their aux counts.  The caller must be holding the
- * queue lock for online tests.
+ * online descendants and their aux counts.
  *
  * If @pol is NULL, blkg_rwstat is at @off bytes into @blkg; otherwise, it
  * is at @off bytes into @blkg's blkg_policy_data of the policy.
@@ -107,7 +106,7 @@ void blkg_rwstat_recursive_sum(struct blkcg_gq *blkg, struct blkcg_policy *pol,
 	blkg_for_each_descendant_pre(pos_blkg, pos_css, blkg) {
 		struct blkg_rwstat *rwstat;
 
-		if (!pos_blkg->online)
+		if (!data_race(pos_blkg->online))
 			continue;
 
 		if (pol) {