blk-cgroup: Remove unnecessary rcu_read_lock/unlock()

spin_lock_irq/spin_unlock_irq contains preempt_disable/enable().
Which can serve as RCU read-side critical region, so remove
rcu_read_lock/unlock().

Signed-off-by: Fanjun Kong <bh1scw@gmail.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220516173930.159535-1-bh1scw@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -1278,14 +1278,13 @@ int blkcg_init_queue(struct request_queue *q)
 	preloaded = !radix_tree_preload(GFP_KERNEL);
 
 	/* Make sure the root blkg exists. */
-	rcu_read_lock();
+	/* spin_lock_irq can serve as RCU read-side critical section. */
 	spin_lock_irq(&q->queue_lock);
 	blkg = blkg_create(&blkcg_root, q, new_blkg);
 	if (IS_ERR(blkg))
 		goto err_unlock;
 	q->root_blkg = blkg;
 	spin_unlock_irq(&q->queue_lock);
-	rcu_read_unlock();
 
 	if (preloaded)
 		radix_tree_preload_end();
@@ -1311,7 +1310,6 @@ err_destroy_all:
 	return ret;
 err_unlock:
 	spin_unlock_irq(&q->queue_lock);
-	rcu_read_unlock();
 	if (preloaded)
 		radix_tree_preload_end();
 	return PTR_ERR(blkg);