rem: return ENOENT;
add: return FS_ENOENT;
context: unremove_vnode(nspace_id nsid, vnode_id vnid){    vnode       *vn;    LOCK(vnlock);    vn = lookup_vnode(nsid, vnid);    if (!vn || (vn->rcnt == 0)) {        UNLOCK(vnlock);        return ENOENT;    }    vn->remove = FALSE;    UNLOCK(vnlock);    return 0;}