md/raid456: convert macro STRIPE_* to RAID5_STRIPE_*

Convert macro STRIPE_SIZE, STRIPE_SECTORS and STRIPE_SHIFT to
RAID5_STRIPE_SIZE(), RAID5_STRIPE_SECTORS() and RAID5_STRIPE_SHIFT().

This patch is prepare for the following adjustable stripe_size.
It will not change any existing functionality.

Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Song Liu <songliubraving@fb.com>