md: factor out helpers for different sync_action in md_do_sync()

Make code cleaner by replacing if else if with switch, and it's more
obvious now what is doing for each sync_action. There are no
functional changes.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240611132251.1967786-11-yukuai1@huaweicloud.com