gre(4): handle IPv6 traffic class like IPv4 tos

[David Gwynne]

pretty straightforward...

ok?

Index: if_gre.c
===================================================================
RCS file: /cvs/src/sys/net/if_gre.c,v
retrieving revision 1.124
diff -u -p -r1.124 if_gre.c
--- if_gre.c    24 Jul 2018 07:40:35 -0000      1.124
+++ if_gre.c    13 Oct 2018 21:57:23 -0000
@@ -1896,11 +1896,20 @@ gre_l3_encap_dst(const struct gre_tunnel
                break;
        }
 #ifdef INET6
-       case AF_INET6:
-               tos = 0;
+       case AF_INET6: {
+               struct ip6_hdr *ip6;
+
+               m = m_pullup(m, sizeof(*ip6));
+               if (m == NULL)
+                       return (NULL);
+
+               ip6 = mtod(m, struct ip6_hdr *);
+               tos = (ntohl(ip6->ip6_flow) & 0x0ff00000) >> 20;
+
                ttloff = offsetof(struct ip6_hdr, ip6_hlim);
                proto = htons(ETHERTYPE_IPV6);
                break;
+       }
  #endif
 #ifdef MPLS
        case AF_MPLS:
@@ -2001,6 +2010,7 @@ gre_encap_dst_ip(const struct gre_tunnel
                ip6->ip6_flow = ISSET(m->m_pkthdr.ph_flowid, M_FLOWID_VALID) ?
                    htonl(m->m_pkthdr.ph_flowid & M_FLOWID_MASK) : 0;
                ip6->ip6_vfc |= IPV6_VERSION;
+               ip6->ip6_flow |= htonl((uint32_t)tos << 20);
                ip6->ip6_plen = htons(len);
                ip6->ip6_nxt = IPPROTO_GRE;
                ip6->ip6_hlim = ttl;