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: 83462a6c

 ("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>