gtp: fix fragmentation needed check with gso

Call skb_gso_validate_network_len() to check if packet is over PMTU.

Fixes: 459aa660 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/gtp.c

@@ -872,8 +872,9 @@ static int gtp_build_skb_ip4(struct sk_buff *skb, struct net_device *dev,
 
 	skb_dst_update_pmtu_no_confirm(skb, mtu);
 
-	if (!skb_is_gso(skb) && (iph->frag_off & htons(IP_DF)) &&
-	    mtu < ntohs(iph->tot_len)) {
+	if (iph->frag_off & htons(IP_DF) &&
+	    ((!skb_is_gso(skb) && skb->len > mtu) ||
+	     (skb_is_gso(skb) && !skb_gso_validate_network_len(skb, mtu)))) {
 		netdev_dbg(dev, "packet too big, fragmentation needed\n");
 		icmp_ndo_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
 			      htonl(mtu));