md: replace sysfs api sync_action with new helpers

To get rid of extrem long if else if usage, and make code cleaner.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240611132251.1967786-6-yukuai1@huaweicloud.com