icmp_do_error
m = icmp_do_error(n, type, code, 0, mtu);
m = icmp_do_error(n, type, code, dest, destmtu);
icmp_do_error(struct mbuf *, int, int, u_int32_t, int);
m = icmp_do_error(m, type, code, 0, destmtu);