vdev_lookup_by_path
if ((mvd = vdev_lookup_by_path(vd->vdev_child[c], path)) !=
vdev_lookup_by_path(rvd, path) == NULL)
newvd = vdev_lookup_by_path(rvd, newpath);
else if (vdev_lookup_by_path(rvd, newpath) != NULL)
vd0 = vdev_lookup_by_path(spa->spa_root_vdev, path0);