Responses for packets to unused ports are getting lost with L3 domains.

IPv4 has ip_send_unicast_reply for sending TCP responses which accounts
for L3 domains; update the IPv6 counterpart tcp_v6_send_response.
For icmp the L3 master check needs to be moved up in icmp6_send
to properly respond to UDP packets to a port with no listener.

Fixes: ca254490 ("net: Add VRF support to IPv6 stack")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>