blkcg: Drop unnecessary RCU read [un]locks from blkg_conf_prep/finish()

blkcg: Drop unnecessary RCU read [un]locks from blkg_conf_prep/finish()

Now that all RCU flavors have been combined either holding a spin lock,
disabling irq or disabling preemption implies RCU read lock, so there's no
need to use rcu_read_[un]lock() explicitly while holding queue_lock. This
shouldn't cause any behavior changes.

v2: Description updated. Leave __acquires/release on queue_lock alone.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230413000649.115785-2-tj@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>