md/raid5: set chunk_sectors to enable full stripe I/O splitting

md/raid5: set chunk_sectors to enable full stripe I/O splitting

Set chunk_sectors to the full stripe width (io_opt) so that the block
layer splits I/O at full stripe boundaries. This ensures that large
writes are aligned to full stripes, avoiding the read-modify-write
overhead that occurs with partial stripe writes in RAID-5/6.

When chunk_sectors is set, the block layer's bio splitting logic in
get_max_io_size() uses blk_boundary_sectors_left() to limit I/O size
to the boundary. This naturally aligns split bios to full stripe
boundaries, enabling more efficient full stripe writes.

Test results with 24-disk RAID5 (chunk_size=64k):
  dd if=/dev/zero of=/dev/md0 bs=10M oflag=direct
  Before: 461 MB/s
  After:  520 MB/s  (+12.8%)

Link: https://lore.kernel.org/linux-raid/20260223035834.3132498-1-yukuai@fnnas.com
Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>