rem: return ENOENT;
add: return FS_ENOENT;
context: put_vnode(nspace_id nsid, vnode_id vnid){    vnode           *vn;    LOCK(vnlock);    vn = lookup_vnode(nsid, vnid);    if (!vn) {        UNLOCK(vnlock);        return ENOENT;    }    UNLOCK(vnlock);    dec_vnode(vn, TRUE);    return 0;}