gtp: really check namespaces before xmit

Blindly assuming that packet transmission crosses namespaces results in
skb marks being lost in the single namespace case.

Signed-off-by: Jonas Bonn <jonas@norrbonn.se>
Acked-by: Harald Welte <laforge@gnumonks.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/gtp.c

@@ -592,7 +592,9 @@ static netdev_tx_t gtp_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 				    ip4_dst_hoplimit(&pktinfo.rt->dst),
 				    0,
 				    pktinfo.gtph_port, pktinfo.gtph_port,
-				    true, false);
+				    !net_eq(sock_net(pktinfo.pctx->sk),
+					    dev_net(dev)),
+				    false);
 		break;
 	}