blk-ioprio: remove per-disk structure

blk-ioprio: remove per-disk structure

ioprio works on the blk-cgroup level, all disks in the same cgroup
are the same, and the struct ioprio_blkg doesn't have anything in it.
Hence register the policy is enough, because cpd_alloc/free_fn will
be handled for each blk-cgroup, and there is no need to activate the
policy for disk. Hence remove blk_ioprio_init/exit and
ioprio_alloc/free_pd.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240719071506.158075-4-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>