rem: return ENOENT;
add: return FS_ENOENT;
context: is_vnode_removed(nspace_id nsid, vnode_id vnid){    vnode       *vn;    int         res;    LOCK(vnlock);    vn = lookup_vnode(nsid, vnid);    if (!vn) {        UNLOCK(vnlock);        return ENOENT;    }    res = vn->remove;    UNLOCK(vnlock);    return res;}