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: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tile/tilepro.c b/drivers/net/tile/tilepro.c
index 0825db6..1e980fd 100644
--- a/drivers/net/tile/tilepro.c
+++ b/drivers/net/tile/tilepro.c
@@ -1930,7 +1930,7 @@
 	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];