net: Use skb_checksum_start_offset()

Replace skb->csum_start - skb_headroom(skb) with skb_checksum_start_offset().

Note for usb/smsc95xx: skb->data - skb->head == skb_headroom(skb).

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/atlx/atl1.c b/drivers/net/atlx/atl1.c
index 5336310..def8df8 100644
--- a/drivers/net/atlx/atl1.c
+++ b/drivers/net/atlx/atl1.c
@@ -2174,7 +2174,7 @@
 	u8 css, cso;
 
 	if (likely(skb->ip_summed == CHECKSUM_PARTIAL)) {
-		css = (u8) (skb->csum_start - skb_headroom(skb));
+		css = skb_checksum_start_offset(skb);
 		cso = css + (u8) skb->csum_offset;
 		if (unlikely(css & 0x1)) {
 			/* L1 hardware requires an even number here */