ipv6: Fix datagram socket connection with DSCP.

Take into account the IPV6_TCLASS socket option (DSCP) in
ip6_datagram_flow_key_init(). Otherwise fib6_rule_match() can't
properly match the DSCP value, resulting in invalid route lookup.

For example:

  ip route add unreachable table main 2001:db8::10/124

  ip route add table 100 2001:db8::10/124 dev eth0
  ip -6 rule add dsfield 0x04 table 100

  echo test | socat - UDP6:[2001:db8::11]:54321,ipv6-tclass=0x04

Without this patch, socat fails at connect() time ("No route to host")
because the fib-rule doesn't jump to table 100 and the lookup ends up
being done in the main table.

Fixes: 2cc67cc7 ("[IPV6] ROUTE: Routing by Traffic Class.")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/datagram.c

@@ -51,7 +51,7 @@ static void ip6_datagram_flow_key_init(struct flowi6 *fl6, struct sock *sk)
 	fl6->flowi6_mark = sk->sk_mark;
 	fl6->fl6_dport = inet->inet_dport;
 	fl6->fl6_sport = inet->inet_sport;
-	fl6->flowlabel = np->flow_label;
+	fl6->flowlabel = ip6_make_flowinfo(np->tclass, np->flow_label);
 	fl6->flowi6_uid = sk->sk_uid;
 
 	if (!oif)