blk-cgroup: don't nest queue_lock under blkcg->lock in blkcg_destroy_blkgs()

blk-cgroup: don't nest queue_lock under blkcg->lock in blkcg_destroy_blkgs()

The correct lock order is q->queue_lock before blkcg->lock, and in order
to prevent deadlock from blkcg_destroy_blkgs(), trylock is used for
q->queue_lock while blkcg->lock is already held, this is hacky.

Refactor blkcg_destroy_blkgs() to hold blkcg->lock only long enough to
get the first blkg and then release it. Then take q->queue_lock and
blkcg->lock in the correct order to destroy the blkg. This is a very cold
path, so the extra lock/unlock cycles are acceptable.

Also prepare to convert protecting blkcg with blkcg_mutex instead of
queue_lock.

Signed-off-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/00b03cf74a9937cb4d6dd67a189ddc00a3de0451.1780621988.git.yukuai@fygo.io
Signed-off-by: Jens Axboe <axboe@kernel.dk>