ospf6d: don't generate Type-7 LSA for route created by "default-information-originate"

ospf6d: don't generate Type-7 LSA for route created by "default-information-originate"

The route created by the "default-information-originate" command
isn't a regular external route. As such, an NSSA ABR shouldn't
originate a corresponding Type-7 LSA for it (there's a separate
configuration knob to generate Type-7 default routes).

While here, fix a small issue in ospf6_asbr_redistribute_add()
where routes created by "default-information-originate" were being
displayed with an incorrect "unknown" type.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>