ipv6: sit: Unmask upper DSCP bits in ipip6_tunnel_bind_dev()

Unmask the upper DSCP bits when calling ip_route_output_ports() so that
in the future it could perform the FIB lookup according to the full DSCP
value.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240903135327.2810535-5-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>