blk-cgroup: delay freeing policy data after rcu grace period

blk-cgroup: delay freeing policy data after rcu grace period

Currently blkcg_print_blkgs() must hold RCU to iterate blkgs from a
blkcg, and prfill() must hold queue_lock to prevent policy data from
being freed by policy deactivation. As a consequence, queue_lock has to
be nested under RCU from blkcg_print_blkgs().

Delay freeing policy data until after an RCU grace period so prfill() can
be protected by RCU alone.

Signed-off-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/e20e5d984b41a026d61851966bed35eb094c4bff.1780621988.git.yukuai@fygo.io
Signed-off-by: Jens Axboe <axboe@kernel.dk>