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

Note that callers of udp_tunnel_dst_lookup() pass the entire DS field in
the 'tos' argument.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>