mpls_output
ifp->if_output = mpls_output;
mpls_output(ifp0, m, &smpls, rt0);
mpls_output(ifp0, m, (struct sockaddr *)&smpls, rt);
mpls_output(ifp0, m0, (struct sockaddr *)&smpls, rt);
int mpls_output(struct ifnet *, struct mbuf *, struct sockaddr *,