blk-cgroup: avoid to warn !rcu_read_lock_held() in blkg_lookup()

So far, all callers either holds spin lock or rcu read explicitly, and
most of the caller has added WARN_ON_ONCE(!rcu_read_lock_held()) or
lockdep_assert_held(&disk->queue->queue_lock).

Remove WARN_ON_ONCE(!rcu_read_lock_held()) from blkg_lookup() for
killing the false positive warning from blkg_conf_prep().

Reported-by: Changhui Zhong <czhong@redhat.com>
Fixes: 83462a6 ("blkcg: Drop unnecessary RCU read [un]locks from blkg_conf_prep/finish()")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20231117023527.3188627-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.h

@@ -249,8 +249,6 @@ static inline struct blkcg_gq *blkg_lookup(struct blkcg *blkcg,
 {
 	struct blkcg_gq *blkg;
 
-	WARN_ON_ONCE(!rcu_read_lock_held());
-
 	if (blkcg == &blkcg_root)
 		return q->root_blkg;