ipv4: Unmask upper DSCP bits in ip_route_output_key_hash()

Unmask the upper DSCP bits so that in the future output routes could be
looked up according to the full DSCP value.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>