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/skbuff.c b/net/core/skbuff.c
index 8814a9a..19d6c21 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1824,7 +1824,7 @@
 	long csstart;
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL)
-		csstart = skb->csum_start - skb_headroom(skb);
+		csstart = skb_checksum_start_offset(skb);
 	else
 		csstart = skb_headlen(skb);