zfs_fuid_idx_domain
domain = zfs_fuid_idx_domain(&idx_tree, FUID_INDEX(id));
char *zfs_fuid_idx_domain(avl_tree_t *, uint32_t);
domain = zfs_fuid_idx_domain(
domain = zfs_fuid_idx_domain(&zfsvfs->z_fuid_idx, idx);