net-ipv6: do not allow IPV6_TCLASS to muck with tcp's ECN

This is to match ipv4 behaviour, see __ip_sock_set_tos()
implementation at ipv4/ip_sockglue.c:579

void __ip_sock_set_tos(struct sock *sk, int val)
{
  if (sk->sk_type == SOCK_STREAM) {
    val &= ~INET_ECN_MASK;
    val |= inet_sk(sk)->tos & INET_ECN_MASK;
  }
  if (inet_sk(sk)->tos != val) {
    inet_sk(sk)->tos = val;
    sk->sk_priority = rt_tos2priority(val);
    sk_dst_reset(sk);
  }
}

Cc: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20211123223154.1117794-1-zenczykowski@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/ipv6_sockglue.c

@@ -599,6 +599,10 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 		/* RFC 3542, 6.5: default traffic class of 0x0 */
 		if (val == -1)
 			val = 0;
+		if (sk->sk_type == SOCK_STREAM) {
+			val &= ~INET_ECN_MASK;
+			val |= np->tclass & INET_ECN_MASK;
+		}
 		np->tclass = val;
 		retv = 0;
 		break;