block: add two helpers for registering/un-registering sched debugfs

Add blk_mq_sched_reg_debugfs()/blk_mq_sched_unreg_debugfs() to clean up
sched init/exit code a bit.

Register & unregister debugfs for sched & sched_hctx order is changed a
bit, but it is safe because sched & sched_hctx is guaranteed to be ready
when exporting via debugfs.

Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250505141805.2751237-6-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>