nbd: fold nbd config initialization into nbd_alloc_config()

nbd: fold nbd config initialization into nbd_alloc_config()

[ Upstream commit 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>
Signed-off-by: Sasha Levin <sashal@kernel.org>