md: add new helpers for sync_action

The new helpers will get current sync_action of the array, will be used
in later patches to 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/20240611132251.1967786-4-yukuai1@huaweicloud.com