openvswitch: use skb_ip_totlen in conntrack

IPv4 GSO packets may get processed in ovs_skb_network_trim(),
and we need to use skb_ip_totlen() to get iph totlen.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>