md/raid5: make sure stripe_size as power of two

Commit 3b5408b9 ("md/raid5: support config stripe_size by sysfs
entry") make stripe_size as a configurable value. It just requires
stripe_size as multiple of 4KB.

In fact, we should make sure stripe_size as power of two. Otherwise,
stripe_shift which is the result of ilog2 can not represent the real
stripe_size. Then, stripe_hash() and stripe_hash_locks_hash() may
get unexpected value.

Fixes: 3b5408b9

 ("md/raid5: support config stripe_size by sysfs entry")
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Song Liu <songliubraving@fb.com>