nbd: factor out a helper to get nbd_config without holding 'config_lock'

Offering: HULK
hulk inclusion
category: bugfix
bugzilla: 188902, https://gitee.com/openeuler/kernel/issues/I7EENU



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There are no functional changes, just to make code cleaner and prepare
to fix null-ptr-dereference while accessing 'nbd->config'.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>