block: add helper add_disk_final()

Add helper add_disk_final() for scanning partitions, announcing disk and
handling the last thing for adding disk.

No functional change, and prepare for prevent adding disk from happening
when updating nr_hw_queues.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250505141805.2751237-8-ming.lei@redhat.com


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