blk-cgroup: delete rcu_read_lock_held() WARN_ON_ONCE()

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: bd1705ce63f2 ("blk-cgroup: Remove unnecessary rcu_read_lock/unlock()")
Signed-off-by: Jens Axboe <axboe@kernel.dk>