drivers: net: use skb_headlen()

replaces (skb->len - skb->data_len) occurrences by skb_headlen(skb)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/atm/he.c b/drivers/atm/he.c
index c213e0d..56c2e99 100644
--- a/drivers/atm/he.c
+++ b/drivers/atm/he.c
@@ -2664,8 +2664,8 @@
 
 #ifdef USE_SCATTERGATHER
 	tpd->iovec[slot].addr = pci_map_single(he_dev->pci_dev, skb->data,
-				skb->len - skb->data_len, PCI_DMA_TODEVICE);
-	tpd->iovec[slot].len = skb->len - skb->data_len;
+				skb_headlen(skb), PCI_DMA_TODEVICE);
+	tpd->iovec[slot].len = skb_headlen(skb);
 	++slot;
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {