Use the onlink flag when adding routes on Linux.

Use the onlink flag when adding routes on Linux.

As reported by iczero, adding gateway routes on Linux can fail if the
gateway address is not reachable yet, either because the interface is
down, or if the gateway address is not inside any route that already is
added or is being added to that interface. By adding the onlink flag,
iproute2 will just add the route without questions.