CVE-2026-90279: md/raid5: round bitmap stripes with sector division

In the Linux kernel, the following vulnerability has been resolved:

md/raid5: round bitmap stripes with sector division

raid5bitmapsectormap() aligns the array range to full RAID5 stripe widths before converting it to component sectors. That width is chunksectors 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 rounddown()/roundup() logic instead gives [1024, 4096), which maps to [0, 1024).

Use sectordiv() 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).