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>

drivers/net/atl1e/atl1e_main.c

@@ -1679,7 +1679,7 @@ static void atl1e_tx_map(struct atl1e_adapter *adapter,
 {
 	struct atl1e_tpd_desc *use_tpd = NULL;
 	struct atl1e_tx_buffer *tx_buffer = NULL;
-	u16 buf_len = skb->len - skb->data_len;
+	u16 buf_len = skb_headlen(skb);
 	u16 map_len = 0;
 	u16 mapped_len = 0;
 	u16 hdr_len = 0;