ipv6: restore correct ECN handling on TCP xmit

Since commit e9df2e8f (Use appropriate sock tclass setting for
routing lookup) we lost ability to properly add ECN codemarks to ipv6
TCP frames.

It seems like TCP_ECN_send() calls INET_ECN_xmit(), which only sets the
ECN bit in the IPv4 ToS field (inet_sk(sk)->tos), but after the patch,
what's checked is inet6_sk(sk)->tclass, which is a completely different
field.

Close bug https://bugzilla.kernel.org/show_bug.cgi?id=34322

[Eric Dumazet] : added the INET_ECN_dontxmit() fix and replace macros
by inline functions for clarity.
Signed-off-by: Steinar H. Gunderson <sgunderson@bigfoot.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_ecn.h

@@ -38,9 +38,19 @@ static inline __u8 INET_ECN_encapsulate(__u8 outer, __u8 inner)
 	return outer;
 }
 
-#define	INET_ECN_xmit(sk) do { inet_sk(sk)->tos |= INET_ECN_ECT_0; } while (0)
-#define	INET_ECN_dontxmit(sk) \
-	do { inet_sk(sk)->tos &= ~INET_ECN_MASK; } while (0)
+static inline void INET_ECN_xmit(struct sock *sk)
+{
+	inet_sk(sk)->tos |= INET_ECN_ECT_0;
+	if (inet6_sk(sk) != NULL)
+		inet6_sk(sk)->tclass |= INET_ECN_ECT_0;
+}
+
+static inline void INET_ECN_dontxmit(struct sock *sk)
+{
+	inet_sk(sk)->tos &= ~INET_ECN_MASK;
+	if (inet6_sk(sk) != NULL)
+		inet6_sk(sk)->tclass &= ~INET_ECN_MASK;
+}
 
 #define IP6_ECN_flow_init(label) do {		\
       (label) &= ~htonl(INET_ECN_MASK << 20);	\