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/route.c b/net/ipv6/route.c
index 194c3cd..2b87418 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1649,7 +1649,7 @@
 	int optlen, on_link;
 	u8 *lladdr;
 
-	optlen = skb->tail - skb->transport_header;
+	optlen = skb_tail_pointer(skb) - skb_transport_header(skb);
 	optlen -= sizeof(*msg);
 
 	if (optlen < 0) {