net:use help function of skb_checksum_start_offset to calculate offset

Although these are equivalent, but the skb_checksum_start_offset() is more readable.
Signed-off-by: NShan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tile/tilepro.c

@@ -1930,7 +1930,7 @@ static int tile_net_tx(struct sk_buff *skb, struct net_device *dev)
 	unsigned int len = skb->len;
 	unsigned char *data = skb->data;
 
-	unsigned int csum_start = skb->csum_start - skb_headroom(skb);
+	unsigned int csum_start = skb_checksum_start_offset(skb);
 
 	lepp_frag_t frags[LEPP_MAX_FRAGS];
 

net/ipv6/udp.c

@@ -1328,7 +1328,7 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb, u32 features)
 	/* 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);