ila: Don't use dest cache when gateway is set

If the gateway is set on an ILA route we don't need to bother with using
the destination cache in the ILA route. Translation does not change the
routing in this case so we can stick with orig_output in the lwstate
output function.

Tested: Ran netperf with and without gateway for LWT route.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>