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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/utils.c

···
 				     __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)));
 	}