net: Use csum_replace_... and csum_sub() helpers instead of opencoding

In a couple places, open coded calculation can be avoided
and replaced by the equivalent csum_replace4() and
csum_replace_by_diff().

There's also one place where csum_sub() should be used instead of
csum_add().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

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)));
 	}

net/netfilter/nft_payload.c

@@ -557,7 +557,7 @@ const struct nft_expr_ops nft_payload_fast_ops = {
 
 static inline void nft_csum_replace(__sum16 *sum, __wsum fsum, __wsum tsum)
 {
-	*sum = csum_fold(csum_add(csum_sub(~csum_unfold(*sum), fsum), tsum));
+	csum_replace4(sum, fsum, tsum);
 	if (*sum == 0)
 		*sum = CSUM_MANGLED_0;
 }