ipv4: make sure RTO_ONLINK is saved in routing cache

__mkroute_output fails to work with the original tos
and uses value with stripped RTO_ONLINK bit. Make sure we put
the original TOS bits into rt_key_tos because it used to match
cached route.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>