ospf6d: do not originate Type-4 into NSSA

ospf6d: do not originate Type-4 into NSSA

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

where r1 is redistributing statics and area 1.1.1.1 is NSSA, the ABR r2 should
not originate type-4 LSA into the NSSA area. From RFC 3101: "NSSA border routers
should not originate Type-4 summary-LSAs into their NSSAs."

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

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