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/ksz884x.c b/drivers/net/ksz884x.c
index 4a231bd..cc0bc8a 100644
--- a/drivers/net/ksz884x.c
+++ b/drivers/net/ksz884x.c
@@ -4684,7 +4684,7 @@
 		int frag;
 		skb_frag_t *this_frag;
 
-		dma_buf->len = skb->len - skb->data_len;
+		dma_buf->len = skb_headlen(skb);
 
 		dma_buf->dma = pci_map_single(
 			hw_priv->pdev, skb->data, dma_buf->len,