inet_ecn: Use csum16_add() helper for IP_ECN_set_* helpers

Jakub pointed out that the IP_ECN_set* helpers basically open-code
csum16_add(), so let's switch them over to using the helper instead.

v2:
- Use __be16 for check_add stack variable in IP_ECN_set_ce() (kbot)
v3:
- Turns out we need __force casts to do arithmetic on __be16 types
Reported-by: Jakub Kicinski <kuba@kernel.org>
Tested-by: Jonathan Morton <chromatix99@gmail.com>
Tested-by: Pete Heist <pete@heistp.net>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20201211142638.154780-1-toke@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/inet_ecn.h

@@ -8,6 +8,7 @@
 
 #include <net/inet_sock.h>
 #include <net/dsfield.h>
+#include <net/checksum.h>
 
 enum {
 	INET_ECN_NOT_ECT = 0,
@@ -75,8 +76,8 @@ static inline void INET_ECN_dontxmit(struct sock *sk)
 
 static inline int IP_ECN_set_ce(struct iphdr *iph)
 {
-	u32 check = (__force u32)iph->check;
 	u32 ecn = (iph->tos + 1) & INET_ECN_MASK;
+	__be16 check_add;
 
 	/*
 	 * After the last operation we have (in binary):
@@ -93,23 +94,20 @@ static inline int IP_ECN_set_ce(struct iphdr *iph)
 	 * INET_ECN_ECT_1 => check += htons(0xFFFD)
 	 * INET_ECN_ECT_0 => check += htons(0xFFFE)
 	 */
-	check += (__force u16)htons(0xFFFB) + (__force u16)htons(ecn);
+	check_add = (__force __be16)((__force u16)htons(0xFFFB) +
+				     (__force u16)htons(ecn));
 
-	iph->check = (__force __sum16)(check + (check>=0xFFFF));
+	iph->check = csum16_add(iph->check, check_add);
 	iph->tos |= INET_ECN_CE;
 	return 1;
 }
 
 static inline int IP_ECN_set_ect1(struct iphdr *iph)
 {
-	u32 check = (__force u32)iph->check;
-
 	if ((iph->tos & INET_ECN_MASK) != INET_ECN_ECT_0)
 		return 0;
 
-	check += (__force u16)htons(0x1);
-
-	iph->check = (__force __sum16)(check + (check>=0xFFFF));
+	iph->check = csum16_add(iph->check, htons(0x1));
 	iph->tos ^= INET_ECN_MASK;
 	return 1;
 }