nbd: fold nbd config initialization into nbd_alloc_config()

Offering: HULK
hulk inclusion
category: bugfix
bugzilla: 188902

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There are no functional changes, make the 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>