selftests/net: add support for PF_RDS sockets

Add support for basic PF_RDS client-server testing in msg_zerocopy

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>