isdn: Correct comparison of 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 the comparison is always between pointers.
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/i4l/isdn_net.c

@@ -885,7 +885,7 @@ isdn_net_log_skb(struct sk_buff *skb, isdn_net_local *lp)
 
 	addinfo[0] = '\0';
 	/* This check stolen from 2.1.72 dev_queue_xmit_nit() */
-	if (p < skb->data || skb->network_header >= skb->tail) {
+	if (p < skb->data || skb_network_header(skb) >= skb_tail_pointer(skb)) {
 		/* fall back to old isdn_net_log_packet method() */
 		char *buf = skb->data;