rem: return ENOMEM;
add: return FS_ENOMEM;
context: sort_vnode(vnode *vn){    if (!InsertSL(skiplist, vn))        return ENOMEM;    vn->nspace.next = vn->ns->vnodes.head;    vn->nspace.prev = NULL;    if (vn->ns->vnodes.head)        vn->ns->vnodes.head->nspace.prev = vn;    else        vn->ns->vnodes.tail = vn;    vn->ns->vnodes.head = vn;    return 0;}