md/raid1: factor out helpers to add rdev to conf

There are no functional changes, just make code cleaner and prepare to
record disk non-rotational information while adding and removing rdev to
conf

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240229095714.926789-3-yukuai1@huaweicloud.com