inet_ecn: Fix endianness of checksum update when setting ECT(1)

When adding support for propagating ECT(1) marking in IP headers it seems I
suffered from endianness-confusion in the checksum update calculation: In
fact the ECN field is in the *lower* bits of the first 16-bit word of the
IP header when calculating in network byte order. This means that the
addition performed to update the checksum field was wrong; let's fix that.

Fixes: b723748 ("tunnel: Propagate ECT(1) when decapsulating as recommended by RFC6040")
Reported-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/20201130183705.17540-1-toke@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/inet_ecn.h

@@ -107,7 +107,7 @@ static inline int IP_ECN_set_ect1(struct iphdr *iph)
 	if ((iph->tos & INET_ECN_MASK) != INET_ECN_ECT_0)
 		return 0;
 
-	check += (__force u16)htons(0x100);
+	check += (__force u16)htons(0x1);
 
 	iph->check = (__force __sum16)(check + (check>=0xFFFF));
 	iph->tos ^= INET_ECN_MASK;