md: wake up 'resync_wait' at last in md_reap_sync_thread()

We just replace md_reap_sync_thread() with wait_event(resync_wait, ...)
from action_store(), this patch just make sure action_store() will still
wait for everything to be done in md_reap_sync_thread().

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230322064122.2384589-6-yukuai1@huaweicloud.com