ip_fragment
error = ip_fragment(m0, ifp, ifp->if_mtu);
error = ip_fragment(m, ifp, ifp->if_mtu);
if (ip_fragment(mb_copy, NULL, mtu) != 0) {
error = ip_fragment(m, ifp, mtu);
int ip_fragment(struct mbuf *, struct ifnet *, u_long);
error = ip_fragment(m, ifp, mtu - sizeof(struct ip));