ospf6d: do not originate Type-4 for NSSA ASBR

ospf6d: do not originate Type-4 for NSSA ASBR

In a topology like:
     r1 ---- 0.0.0.0 ---- r2(ABR) ---- 1.1.1.1 -----r3(ASBR)

where r3 is redistributing statics and area 1.1.1.1 is NSSA, the ABR r2 should
not originate type-4 LSA, according to RFC 3101, section 1.3:

"also an NSSA's border routers never originate Type-4 summary-LSAs for the
NSSA's AS boundary routers, since Type-7 AS-external-LSAs are never flooded
beyond the NSSA's border"

r1# sh ipv6 os database inter-router

        Area Scoped Link State Database (Area 0.0.0.0)

Type LSId           AdvRouter       Age   SeqNum                        Payload
IAR  3.3.3.3        2.2.2.2          49 80000001                        3.3.3.3

This PR prevents the above LSA of being originated by the ABR r2

Signed-off-by: ckishimo <carles.kishimoto@gmail.com>