inet: Make tunnel RX/TX byte counters more consistent

This patch makes the RX/TX byte counters for IPIP, GRE and SIT more
consistent.  Previously we included the external IP headers on the
way out but not when the packet is inbound.

The new scheme is to count payload only in both directions.  For
IPIP and SIT this simply means the exclusion of the external IP
header.  For GRE this means that we exclude the GRE header as
well.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ipip.h b/include/net/ipip.h
index a85bda6..fdf9bd7 100644
--- a/include/net/ipip.h
+++ b/include/net/ipip.h
@@ -37,7 +37,7 @@
 
 #define IPTUNNEL_XMIT() do {						\
 	int err;							\
-	int pkt_len = skb->len;						\
+	int pkt_len = skb->len - skb_transport_offset(skb);		\
 									\
 	skb->ip_summed = CHECKSUM_NONE;					\
 	ip_select_ident(iph, &rt->u.dst, NULL);				\