md: factor out a helper from mddev_put()

commit 3d8d3287 upstream.

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


[minor context conflict]
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Cc: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>