gtp: clear DF bit on GTP packet tx

3GPP TS 29.281 and 3GPP TS 29.060 imply that GTP-U packets should be
sent with the DF bit cleared. For example 3GPP TS 29.060, Release 8,
Section 13.2.2:

> Backbone router: Any router in the backbone may fragment the GTP
> packet if needed, according to IPv4.
Signed-off-by: NAndreas Schultz <aschultz@tpip.net>
Acked-by: NHarald Welte <laforge@netfilter.org>
Acked-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/gtp.c

@@ -612,7 +612,7 @@ static netdev_tx_t gtp_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 				    pktinfo.fl4.saddr, pktinfo.fl4.daddr,
 				    pktinfo.iph->tos,
 				    ip4_dst_hoplimit(&pktinfo.rt->dst),
-				    htons(IP_DF),
+				    0,
 				    pktinfo.gtph_port, pktinfo.gtph_port,
 				    true, false);
 		break;