vpn: add route to vpn gw when parent has a default device route
When the parent device has a device default route (i.e. without gateway) and we establish a VPN on top of it, 'ip route get' for the VPN gateway returns a device route, which is the same result we get for an unreachable VPN gateway. However it is necessary to add the route to the gateway or otherwise it will possibly become unreachable once the VPN gets activated.