blk-cgroup: don't nest queue_lock under rcu in bio_associate_blkg()

blk-cgroup: don't nest queue_lock under rcu in bio_associate_blkg()

If a bio is already associated with a blkg, the blkcg is already pinned
until the bio is done, so there is no need for RCU protection. Otherwise,
protect blkcg_css() with RCU independently. Prepare to protect blkcg with
blkcg_mutex instead of queue_lock.

Signed-off-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/8496fa234b21d4b31b7f068766906d0bffcac8e6.1780621988.git.yukuai@fygo.io
Signed-off-by: Jens Axboe <axboe@kernel.dk>