md: record dm-raid gendisk in mddev

Following patch will use gendisk to check if there are normal IO
completed or inflight, to fix a problem in mdraid that foreground IO
can be starved by background sync IO in later patches.

Link: https://lore.kernel.org/linux-raid/20250506124903.2540268-7-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>