blk-cgroup: delete rcu_read_lock_held() WARN_ON_ONCE()

A previous commit got rid of unnecessary rcu_read_lock() inside the
IRQ disabling queue_lock, but this debug statement was left. It's now
firing since we are indeed not inside a RCU read lock, but we don't
need to be as we're still preempt safe.

Get rid of the check, as we have a lockdep assert for holding the
queue lock right after it anyway.

Link: https://lore.kernel.org/linux-block/46253c48-81cb-0787-20ad-9133afdd9e21@samsung.com/
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: 77c570a ("blk-cgroup: Remove unnecessary rcu_read_lock/unlock()")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -298,7 +298,6 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
 	struct blkcg_gq *blkg;
 	int i, ret;
 
-	WARN_ON_ONCE(!rcu_read_lock_held());
 	lockdep_assert_held(&q->queue_lock);
 
 	/* request_queue is dying, do not create/recreate a blkg */