block: move bd_part_count to struct gendisk

The bd_part_count value only makes sense for whole devices, so move it
to struct gendisk and give it a more descriptive name.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210525061301.2242282-7-hch@lst.de


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