nbd: nbd_reset() call in nbd_dev_add() is redundant

There is nothing to clear -- nbd_device has just been allocated.
Fold nbd_reset() into its other caller, nbd_config_put().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>