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;
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);
 
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index b37181d..1198adf 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2226,7 +2226,7 @@
 	/* Do software UFO. Complete and fill in the UDP checksum as HW cannot
 	 * do checksum of UDP packets sent as multiple IP fragments.
 	 */
-	offset = skb->csum_start - skb_headroom(skb);
+	offset = skb_checksum_start_offset(skb);
 	csum = skb_checksum(skb, offset, skb->len - offset, 0);
 	offset += skb->csum_offset;
 	*(__sum16 *)(skb->data + offset) = csum_fold(csum);
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index e79efaf..91cb1d7 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1650,8 +1650,7 @@
 
 		if (skb->ip_summed == CHECKSUM_PARTIAL) {
 			vnet_hdr.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
-			vnet_hdr.csum_start = skb->csum_start -
-							skb_headroom(skb);
+			vnet_hdr.csum_start = skb_checksum_start_offset(skb);
 			vnet_hdr.csum_offset = skb->csum_offset;
 		} /* else everything is zero */