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/e1000/e1000_main.c b/drivers/net/e1000/e1000_main.c
index 47da5fc..974a02d 100644
--- a/drivers/net/e1000/e1000_main.c
+++ b/drivers/net/e1000/e1000_main.c
@@ -2929,7 +2929,7 @@
 	unsigned int first, max_per_txd = E1000_MAX_DATA_PER_TXD;
 	unsigned int max_txd_pwr = E1000_MAX_TXD_PWR;
 	unsigned int tx_flags = 0;
-	unsigned int len = skb->len - skb->data_len;
+	unsigned int len = skb_headlen(skb);
 	unsigned int nr_frags;
 	unsigned int mss;
 	int count = 0;
@@ -2980,7 +2980,7 @@
 					dev_kfree_skb_any(skb);
 					return NETDEV_TX_OK;
 				}
-				len = skb->len - skb->data_len;
+				len = skb_headlen(skb);
 				break;
 			default:
 				/* do nothing */