[NET]: Use pskb_expand_head() instead of skb_copy() in skb_checksum_help().

Here is the patch that you wanted to shoot holes at :)

The idea is simple.  None of the callers of skb_checksum are passing it
skb's which are shared.  They may be cloned however.  But the application
checksum is always in the skb header so there is no need to linearise it.

Supposing all these assumptions are correct, then we can avoid the overhead
of skb_copy() and get away with pskb_expand_head().

If the assumption is wrong, we will find out because
pskb_expand_head() will BUG() if skb_shared() is true.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@redhat.com>

net/core/dev.c

@@ -1144,16 +1144,10 @@ int skb_checksum_help(struct sk_buff **pskb, int inward)
 		goto out;
 	}
 
-	if (skb_shared(*pskb)  || skb_cloned(*pskb)) {
-		struct sk_buff *newskb = skb_copy(*pskb, GFP_ATOMIC);
-		if (!newskb) {
-			ret = -ENOMEM;
+	if (skb_cloned(*pskb)) {
+		ret = pskb_expand_head(*pskb, 0, 0, GFP_ATOMIC);
+		if (ret)
 			goto out;
-		}
-		if ((*pskb)->sk)
-			skb_set_owner_w(newskb, (*pskb)->sk);
-		kfree_skb(*pskb);
-		*pskb = newskb;
 	}
 
 	if (offset > (int)(*pskb)->len)