blk-cgroup: fix rcu lockdep warning in blkg_lookup()

blkg_lookup() is called with either queue_lock or rcu read lock, so
use rcu_dereference_check(lockdep_is_held(&q->queue_lock)) for
retrieving 'blkg', which way models the check exactly for covering
queue lock or rcu read lock.

Fix lockdep warning of "block/blk-cgroup.h:254 suspicious rcu_dereference_check() usage!"
from blkg_lookup().

Tested-by: Changhui Zhong <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>
Fixes: 83462a6c97 ("blkcg: Drop unnecessary RCU read [un]locks from blkg_conf_prep/finish()")
Acked-by: Tejun Heo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

block/blk-cgroup.h

@@ -252,7 +252,8 @@ static inline struct blkcg_gq *blkg_lookup(struct blkcg *blkcg,
 	if (blkcg == &blkcg_root)
 		return q->root_blkg;
 
-	blkg = rcu_dereference(blkcg->blkg_hint);
+	blkg = rcu_dereference_check(blkcg->blkg_hint,
+			lockdep_is_held(&q->queue_lock));
 	if (blkg && blkg->q == q)
 		return blkg;