netfilter: ipvs: full-functionality option for ECN encapsulation in tunnel

IPVS tunnel mode works as simple tunnel (see RFC 3168) copying ECN field
to outer header. That's result in packet drops on egress tunnels in case
the egress tunnel operates as ECN-capable with Full-functionality option
(like ip_tunnel and ip6_tunnel kernel modules), according to RFC 3168
section 9.1.1 recommendation.

This patch implements ECN full-functionality option into ipvs xmit code.

Cc: netdev@vger.kernel.org
Cc: lvs-devel@vger.kernel.org
Signed-off-by: Vadim Fedorenko <vfedorenko@yandex-team.ru>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>