block: move adjusting bd_part_count out of __blkdev_get

Keep in the callers and thus remove the for_part argument.  This mirrors
what is done on the blkdev_get side and slightly simplifies
blkdev_get_part as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@rehat.com>
Link: https://lore.kernel.org/r/20210525061301.2242282-5-hch@lst.de


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