md/raid5: fix previous patch.

Previous patch changes stripe and chunk_number to sector_t but
mistakenly did not update all of the divisions to use sector_dev().

This patch changes all the those divisions (actually the '%' operator)
to sector_div.

Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org
Tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>