net: Correct comparisons and calculations using skb->tail and skb-transport_header

This corrects an regression introduced by "net: Use 16bits for *_headers
fields of struct skbuff" when NET_SKBUFF_DATA_USES_OFFSET is not set. In
that case skb->tail will be a pointer whereas skb->transport_header
will be an offset from head. This is corrected by using wrappers that
ensure that comparisons and calculations are always made using pointers.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/inet_ecn.h b/include/net/inet_ecn.h
index aab7375..3bd2279 100644
--- a/include/net/inet_ecn.h
+++ b/include/net/inet_ecn.h
@@ -134,12 +134,14 @@
 {
 	switch (skb->protocol) {
 	case cpu_to_be16(ETH_P_IP):
-		if (skb->network_header + sizeof(struct iphdr) <= skb->tail)
+		if (skb_network_header(skb) + sizeof(struct iphdr) <=
+		    skb_tail_pointer(skb))
 			return IP_ECN_set_ce(ip_hdr(skb));
 		break;
 
 	case cpu_to_be16(ETH_P_IPV6):
-		if (skb->network_header + sizeof(struct ipv6hdr) <= skb->tail)
+		if (skb_network_header(skb) + sizeof(struct ipv6hdr) <=
+		    skb_tail_pointer(skb))
 			return IP6_ECN_set_ce(ipv6_hdr(skb));
 		break;
 	}