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