vdev_config_sync
error = vdev_config_sync(svd, svdcount, spa->spa_first_txg);
error = vdev_config_sync(svd, svdcount, txg);
error = vdev_config_sync(rvd->vdev_child,
extern int vdev_config_sync(vdev_t **svd, int svdcount, uint64_t txg);