blk-cgroup: delay calling blkcg_exit_disk until disk_release

blk-cgroup: delay calling blkcg_exit_disk until disk_release

While del_gendisk ensures there is no outstanding I/O on the queue,
it can't prevent block layer users from building new I/O.

This leads to a NULL ->root_blkg reference in bio_associate_blkg when
allocating a new bio on a shut down file system.  Delay freeing the
blk-cgroup subsystems from del_gendisk until disk_release to make
sure the blkg and throttle information is still avaіlable for bio
submitters, even if those bios will immediately fail.

This now can cause a case where disk_release is called on a disk
that hasn't been added.  That's mostly harmless, except for a case
in blk_throttl_exit that now needs to check for a NULL ->td pointer.

Fixes: 178fa7d49815 ("blk-cgroup: delay blk-cgroup initialization until add_disk")
Reported-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230208063514.171485-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>