dnsmasq has a bug that it sends DHCPOffer packet via wrong interface if it is dhcp relay mode

fix-correct-interface-if-relay-mode.patch

--- a/src/dhcp.c  2017-05-23 06:58:46.000000000 +0900
+++ b/src/dhcp.c  2017-09-15 14:31:43.586293892 +0900
@@ -399,7 +399,7 @@
       msg.msg_controllen = sizeof(control_u);
       cmptr = CMSG_FIRSTHDR(&msg);
       pkt = (struct in_pktinfo *)CMSG_DATA(cmptr);
-      pkt->ipi_ifindex = rcvd_iface_index;
+      pkt->ipi_ifindex = is_relay_reply ? iface_index : rcvd_iface_index;
       pkt->ipi_spec_dst.s_addr = 0;
       msg.msg_controllen = cmptr->cmsg_len = CMSG_LEN(sizeof(struct in_pktinfo));
       cmptr->cmsg_level = IPPROTO_IP;