md: add a new api prepare_suspend() in md_personality

There are no functional changes, the new api will be used later to do
special handling for raid456 in md_suspend().

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230512015610.821290-5-yukuai1@huaweicloud.com