ospf6d: always generate default route for stubs

ospf6d: always generate default route for stubs

In RFC 2328 OSPF Version 2, Section 12.4.3.1 "Originating summary-LSAs
into stub areas" mentions that the stub areas should not import external
routes and instead should generate a 'default summary-LSA' set to
default destination.

> In a stub area, instead of importing external routes
> each area border router originates a "default summary-
> LSA" into the area. The Link State ID for the default
> summary-LSA is set to DefaultDestination, and the metric
> set to the (per-area) configurable parameter
> StubDefaultCost.  Note that StubDefaultCost need not be
> configured identically in all of the stub area's area
> border routers.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>