iptunnel: make rx/tx bytes counters consistent

This was already done a long time ago in
commit 64194c31 ("inet: Make tunnel RX/TX byte counters more consistent")
but tx path was broken (at least since 3.10).

Before the patch the gre header was included on tx.

After the patch:
$ ping -c1 192.168.0.121 ; ip -s l ls dev gre1
PING 192.168.0.121 (192.168.0.121) 56(84) bytes of data.
64 bytes from 192.168.0.121: icmp_req=1 ttl=64 time=2.95 ms

--- 192.168.0.121 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 2.955/2.955/2.955/0.000 ms
7: gre1@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1468 qdisc noqueue state UNKNOWN mode DEFAULT group default
    link/gre 10.16.0.249 peer 10.16.0.121
    RX: bytes  packets  errors  dropped overrun mcast
    84         1        0       0       0       0
    TX: bytes  packets  errors  dropped carrier collsns
    84         1        0       0       0       0
Reported-by: Julien Meunier <julien.meunier@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel_core.c

@@ -51,7 +51,7 @@ int iptunnel_xmit(struct sock *sk, struct rtable *rt, struct sk_buff *skb,
 		  __be32 src, __be32 dst, __u8 proto,
 		  __u8 tos, __u8 ttl, __be16 df, bool xnet)
 {
-	int pkt_len = skb->len;
+	int pkt_len = skb->len - skb_inner_network_offset(skb);
 	struct iphdr *iph;
 	int err;