blk-mq: factor out some helps to quiesce/unquiesce queue

hulk inclusion
category: bugfix
bugzilla: 173974
CVE: NA
---------------------------

Prepare to support concurrent quiesce queue between drivers and block
layer, no functional changes.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>