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/net/core/dev.c b/net/core/dev.c
index 794b20d..92d414a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1793,7 +1793,7 @@
 		goto out_set_summed;
 	}
 
-	offset = skb->csum_start - skb_headroom(skb);
+	offset = skb_checksum_start_offset(skb);
 	BUG_ON(offset >= skb_headlen(skb));
 	csum = skb_checksum(skb, offset, skb->len - offset, 0);
 
@@ -2090,8 +2090,8 @@
 			 * checksumming here.
 			 */
 			if (skb->ip_summed == CHECKSUM_PARTIAL) {
-				skb_set_transport_header(skb, skb->csum_start -
-					      skb_headroom(skb));
+				skb_set_transport_header(skb,
+					skb_checksum_start_offset(skb));
 				if (!dev_can_checksum(dev, skb) &&
 				     skb_checksum_help(skb))
 					goto out_kfree_skb;