ospfd: fix ospf default route wrongly sent back

ospfd: fix ospf default route wrongly sent back

That commit aim is to fix an invalid behavior when
default-information is activated on ospf router without always option.

Consider an ASBR with:
-one default route coming from ospf,
-and another default route coming from another deaemon (such BGP or static).

When the daemon bgp stops advertising its default route,
-ospf continues to advertise its previous default route (with aging 0),
-this may create default routing loops.

Expected behavior: is to update the removed external default route with
MAXAGING value.
Updating with MAXAGING value will notify the fact the route is currently
invalid. A later removal from ospf external LSA database will be made.

Analysis: all default routes have their type overwritten by a
DEFAULT_ROUTE type. Thus all default routes whatever its origin (ospf,
bgp, static...) is treated in a same way. But this is not pertinent for
ospf originated default routes.

Fix: avoid overwiting of route type when default route is ospf type.

Signed-off-by: Francois Dumontet <francois.dumontet@6wind.com>