block: reuse part_in_flight_rw for part_in_flight

They are almost identical, to make code cleaner.

Link: https://lore.kernel.org/linux-raid/20250506124903.2540268-2-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>