hammer2_get_volume_attr
if (hammer2_get_volume_attr(offset, hammer2_volume_fd_cb, &ret) < 0)
if (hammer2_get_volume_attr(offset, hammer2_volume_id_cb, &ret) < 0)
if (hammer2_get_volume_attr(offset, hammer2_volume_path_cb, &ret) < 0)
if (hammer2_get_volume_attr(offset, hammer2_volume_offset_cb, &ret) < 0)
if (hammer2_get_volume_attr(offset, hammer2_volume_size_cb, &ret) < 0)