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/net/benet/be_main.c b/drivers/net/benet/be_main.c
index 18e0a80..fa10f13 100644
--- a/drivers/net/benet/be_main.c
+++ b/drivers/net/benet/be_main.c
@@ -432,7 +432,7 @@
 	map_head = txq->head;
 
 	if (skb->len > skb->data_len) {
-		int len = skb->len - skb->data_len;
+		int len = skb_headlen(skb);
 		busaddr = pci_map_single(pdev, skb->data, len,
 					 PCI_DMA_TODEVICE);
 		if (pci_dma_mapping_error(pdev, busaddr))
@@ -1098,7 +1098,7 @@
 		cur_index = txq->tail;
 		wrb = queue_tail_node(txq);
 		unmap_tx_frag(adapter->pdev, wrb, (unmap_skb_hdr &&
-					sent_skb->len > sent_skb->data_len));
+					skb_headlen(sent_skb)));
 		unmap_skb_hdr = false;
 
 		num_wrbs++;