net: core: Use csum_replace_by_diff() and csum_sub() instead of opencoding

Open coded calculation can be avoided and replaced by the
equivalent csum_replace_by_diff() and csum_sub().
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/utils.c

@@ -476,9 +476,9 @@ void inet_proto_csum_replace_by_diff(__sum16 *sum, struct sk_buff *skb,
 				     __wsum diff, bool pseudohdr)
 {
 	if (skb->ip_summed != CHECKSUM_PARTIAL) {
-		*sum = csum_fold(csum_add(diff, ~csum_unfold(*sum)));
+		csum_replace_by_diff(sum, diff);
 		if (skb->ip_summed == CHECKSUM_COMPLETE && pseudohdr)
-			skb->csum = ~csum_add(diff, ~skb->csum);
+			skb->csum = ~csum_sub(diff, skb->csum);
 	} else if (pseudohdr) {
 		*sum = ~csum_fold(csum_add(diff, csum_unfold(*sum)));
 	}