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>

net/openvswitch/conntrack.c

@@ -1103,7 +1103,7 @@ static int ovs_skb_network_trim(struct sk_buff *skb)
 
 	switch (skb->protocol) {
 	case htons(ETH_P_IP):
-		len = ntohs(ip_hdr(skb)->tot_len);
+		len = skb_ip_totlen(skb);
 		break;
 	case htons(ETH_P_IPV6):
 		len = sizeof(struct ipv6hdr)