cxgb3: 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.

Cc: Simon Horman <horms@verge.net.au>
Signed-off-by: NLi RongQing <roy.qing.li@gmail.com>
Reviewed-by: NSimon Horman <horms@verge.net.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb3/sge.c

@@ -1583,7 +1583,7 @@ static void deferred_unmap_destructor(struct sk_buff *skb)
 	dui = (struct deferred_unmap_info *)skb->head;
 	p = dui->addr;
 
-	if (skb->tail - skb->transport_header)
+	if (skb_tail_pointer(skb) - skb_transport_header(skb))
 		pci_unmap_single(dui->pdev, *p++, skb_tail_pointer(skb) -
 				 skb_transport_header(skb), PCI_DMA_TODEVICE);
 

drivers/net/ethernet/chelsio/cxgb4/sge.c

@@ -1294,7 +1294,7 @@ static inline unsigned int calc_tx_flits_ofld(const struct sk_buff *skb)
 
 	flits = skb_transport_offset(skb) / 8U;   /* headers */
 	cnt = skb_shinfo(skb)->nr_frags;
-	if (skb->tail != skb->transport_header)
+	if (skb_tail_pointer(skb) != skb_transport_header(skb))
 		cnt++;
 	return flits + sgl_len(cnt);
 }