md: use READ_ONCE/WRITE_ONCE for 'suspend_lo' and 'suspend_hi'

Protect 'suspend_lo' and 'suspend_hi' with READ_ONCE/WRITE_ONCE to prevent
reading abnormal values.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231010151958.145896-2-yukuai1@huaweicloud.com