block: factor out a new helper from blk_rq_init()

hulk inclusion
category: bugfix
bugzilla: 34280, https://gitee.com/openeuler/kernel/issues/I4AKY4


CVE: NA

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Prepare to fix kernel panic during iterating over flush request, 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>