udp: honor IPV6_TCLASS cmsg for UDP/IPv4 packets

Honor the IPPROTO_IPV6-level cmsg of type IPV6_TCLASS when sending
an UDP/IPv4 packet on an AF_INET6 socket.

Reviewed by:		bz
MFC after:		3 days
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D53347

sys/netinet/udp_usrreq.c

@@ -1208,6 +1208,23 @@ udp_send(struct socket *so, int flags, struct mbuf *m, struct sockaddr *addr,
 			error = udp_v4mapped_pktinfo(cm, &src, inp, flags);
 			if (error != 0)
 				break;
+			if (((flags & PRUS_IPV6) != 0) &&
+			    (cm->cmsg_level == IPPROTO_IPV6) &&
+			    (cm->cmsg_type == IPV6_TCLASS)) {
+				int tclass;
+
+				if (cm->cmsg_len != CMSG_LEN(sizeof(int))) {
+					error = EINVAL;
+					break;
+				}
+				tclass = *(int *)CMSG_DATA(cm);
+				if (tclass < -1 || tclass > 255) {
+					error = EINVAL;
+					break;
+				}
+				if (tclass != -1)
+					tos = (u_char)tclass;
+			}
 #endif
 			if (cm->cmsg_level != IPPROTO_IP)
 				continue;