blk-cgroup: protect iterating blkgs with blkcg->lock in blkcg_print_stat()

blk-cgroup: protect iterating blkgs with blkcg->lock in blkcg_print_stat()

blkcg_print_one_stat() will be called for each blkg:
- access blkg->iostat, which is freed from rcu callback
  blkg_free_workfn();
- access policy data from pd_stat_fn(), which is freed from
  pd_free_fn(), while pd_free_fn() can be called by removing blkcg or
  deactivating policy;

Take blkcg->lock while iterating so the blkgs stay online and both
blkg->iostat and policy data for activated policies stay valid.  Use
irq-safe locking because blkcg->lock can be nested under q->queue_lock,
which is used from IRQ completion paths.

Prepare to convert protecting blkgs from request_queue with mutex.

Signed-off-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/05799877e720dcd300e2ddd4625e8e162959d7cc.1780621988.git.yukuai@fygo.io
Signed-off-by: Jens Axboe <axboe@kernel.dk>