md: use new apis to suspend array for sysfs apis

Convert to use new apis in following sysfs apis:
 - level_store
 - suspend_lo_store
 - suspend_hi_store
 - serialize_policy_store

These are not hot path, so performance is not concerned.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231010151958.145896-11-yukuai1@huaweicloud.com