ip6_tunnel: fix traffic class routing for tunnels

ip6_route_output() requires that the flowlabel contains the traffic
class for policy routing.

Commit 0e9a7095 ("ip6_tunnel, ip6_gre: fix setting of DSCP on
encapsulated packets") removed the code which previously added the
traffic class to the flowlabel.

The traffic class is added here because only route lookup needs the
flowlabel to contain the traffic class.

Fixes: 0e9a7095

 ("ip6_tunnel, ip6_gre: fix setting of DSCP on encapsulated packets")
Signed-off-by: Liam McBirnie <liam.mcbirnie@boeing.com>
Acked-by: Peter Dawson <peter.a.dawson@boeing.com>
Signed-off-by: David S. Miller <davem@davemloft.net>