md: use new helpers in md_do_sync()

Make code cleaner. and also use the action_name directly in kernel log:
 - "check" instead of "data-check"
 - "repair" instead of "requested-resync"

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240611132251.1967786-9-yukuai1@huaweicloud.com