Make sure we set the key in the linkstate route entry

Make sure we set the key in the linkstate route entry

This fixes a missing key when storing the link state route. In order to
make it efficient, we need to pass back the InstanceIdentifier which
will be holding that entry. This also has the nice property of
increasing efficiency for IPv4/IPv6 routes by reusing the route key.

Change-Id: I777594feb283a7e5874da5cf8eaf6a97af49f1ab
Signed-off-by: Robert Varga <rovarga@cisco.com>