ovl_setup_trap
err = ovl_setup_trap(sb, l->path.dentry, &trap, "lowerdir");
err = ovl_setup_trap(sb, upperpath->dentry, &upper_layer->trap,
err = ovl_setup_trap(sb, ofs->workdir, &ofs->workdir_trap, "workdir");
err = ovl_setup_trap(sb, ofs->workbasedir, &ofs->workbasedir_trap,
err = ovl_setup_trap(sb, indexdir, &ofs->workdir_trap,