md: factor out a helper from mddev_put()

There are no functional changes, prepare to simplify md_seq_ops in next
patch.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230927061241.1552837-2-yukuai1@huaweicloud.com