xen-netfront: use skb_partial_csum_set() to simplify the codes

use skb_partial_csum_set() to simplify the codes

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -858,7 +858,6 @@ static RING_IDX xennet_fill_frags(struct netfront_info *np,
 static int checksum_setup(struct net_device *dev, struct sk_buff *skb)
 {
 	struct iphdr *iph;
-	unsigned char *th;
 	int err = -EPROTO;
 	int recalculate_partial_csum = 0;
 
@@ -883,27 +882,27 @@ static int checksum_setup(struct net_device *dev, struct sk_buff *skb)
 		goto out;
 
 	iph = (void *)skb->data;
-	th = skb->data + 4 * iph->ihl;
-	if (th >= skb_tail_pointer(skb))
-		goto out;
 
-	skb->csum_start = th - skb->head;
 	switch (iph->protocol) {
 	case IPPROTO_TCP:
-		skb->csum_offset = offsetof(struct tcphdr, check);
+		if (!skb_partial_csum_set(skb, 4 * iph->ihl,
+					  offsetof(struct tcphdr, check)))
+			goto out;
 
 		if (recalculate_partial_csum) {
-			struct tcphdr *tcph = (struct tcphdr *)th;
+			struct tcphdr *tcph = tcp_hdr(skb);
 			tcph->check = ~csum_tcpudp_magic(iph->saddr, iph->daddr,
 							 skb->len - iph->ihl*4,
 							 IPPROTO_TCP, 0);
 		}
 		break;
 	case IPPROTO_UDP:
-		skb->csum_offset = offsetof(struct udphdr, check);
+		if (!skb_partial_csum_set(skb, 4 * iph->ihl,
+					  offsetof(struct udphdr, check)))
+			goto out;
 
 		if (recalculate_partial_csum) {
-			struct udphdr *udph = (struct udphdr *)th;
+			struct udphdr *udph = udp_hdr(skb);
 			udph->check = ~csum_tcpudp_magic(iph->saddr, iph->daddr,
 							 skb->len - iph->ihl*4,
 							 IPPROTO_UDP, 0);
@@ -917,9 +916,6 @@ static int checksum_setup(struct net_device *dev, struct sk_buff *skb)
 		goto out;
 	}
 
-	if ((th + skb->csum_offset + 2) > skb_tail_pointer(skb))
-		goto out;
-
 	err = 0;
 
 out: