ipv6: 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/net/ipv6/icmp.c b/net/ipv6/icmp.c
index 1d2902e..4b4890b 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -399,7 +399,7 @@
 	int err = 0;
 
 	if ((u8 *)hdr < skb->head ||
-	    (skb->network_header + sizeof(*hdr)) > skb->tail)
+	    (skb_network_header(skb) + sizeof(*hdr)) > skb_tail_pointer(skb))
 		return;
 
 	/*