md: replace last_sync_action with new enum type

The only difference is that "none" is removed and initial
last_sync_action will be idle.

On the one hand, this value is introduced by commit c4a39551
("MD: Remember the last sync operation that was performed"), and the
usage described in commit message is not affected. On the other hand,
last_sync_action is not used in mdadm or mdmon, and none of the tests
that I can find.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240611132251.1967786-10-yukuai1@huaweicloud.com