md/raid5: round bitmap stripes with sector division

raid5_bitmap_sector_map() aligns the array range to full RAID5 stripe
widths before converting it to component sectors. That width is
chunk_sectors multiplied by the number of data disks, and it is not
always a power of two.

Reproduce with a 4-disk RAID5, 1024-sector chunks, and three data disks.
The full-stripe width is 3072 sectors. For a one-sector write at array
sector 3072, correct rounding gives array range [3072, 6144), which maps
to component range [1024, 2048). The old round_down()/round_up() logic
instead gives [1024, 4096), which maps to [0, 1024).

Use sector_div() based arithmetic so the rounded range is aligned to the
actual RAID5 stripe width.

The deterministic mapper test now reports the fixed component range as
[1024, 2048), while the old mask-based range was [0, 1024).

Fixes: 9c89f604476c ("md/raid5: implement pers->bitmap_sector()")
Reported-by: Mykola Marzhan <mykola@meshstor.io>
Link: https://lore.kernel.org/all/20260726185916.2223460-1-mykola@meshstor.io/
Tested-by: Mykola Marzhan <mykola@meshstor.io>
Link: https://patch.msgid.link/20260802195038.164272-6-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai@fygo.io>

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