nbd: don't leak nbd_config

nbd_config is allocated in nbd_alloc_config(), but never freed.

Fixes: 5ea8d108 ("nbd: separate out the config information")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>