block: clean up blk_mq_in_flight_rw()

Also add comment for part_inflight_show() for the difference between
bio-based and rq-based device.

Link: https://lore.kernel.org/linux-raid/20250506124903.2540268-4-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>