md/raid5: support config stripe_size by sysfs entry

Adding a new 'stripe_size' sysfs entry to set and show stripe_size.
stripe_size should not be bigger than PAGE_SIZE, and it requires to
be multiple of 4096. We can adjust stripe_size by writing value into
sysfs entry, likely, set stripe_size as 16KB:

          echo 16384 > /sys/block/md1/md/stripe_size

Show current stripe_size value:

          cat /sys/block/md1/md/stripe_size

For PAGE_SIZE is equal to 4096, 'stripe_size' can just be read.

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