nbd: fold nbd config initialization into nbd_alloc_config()

mainline inclusion
from mainline-v6.7-rc3
commit 1b59860540a4018e8071dc18d4893ec389506b7d
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8P7B3

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1b59860540a4018e8071dc18d4893ec389506b7d



--------------------------------

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

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20231116162316.1740402-2-linan666@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>