udp: honor IPV6_TCLASS socket option for UDP/IPv4 packets
Honor the IPPROTO_IPV6-level socket option IPV6_TCLASS when sending
an UDP/IPv4 packet on an AF_INET6 socket.
Reviewed by: bz, glebius
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D53346
(cherry picked from commit 3535546a86846ddb0ca5fe4a0689ac635b504459)