dbg_walk_index
err = dbg_walk_index(c, NULL, add_size, &calc);
err = dbg_walk_index(c, check_leaf, NULL, &fsckd);
dbg_walk_index(c, NULL, dump_znode, NULL);
int dbg_walk_index(struct ubifs_info *c, dbg_leaf_callback leaf_cb,
err = dbg_walk_index(c, &dbg_orphan_check, NULL, &ci);