ospfd: do not generate type 4 LSA from NSSA ABR

ospfd: do not generate type 4 LSA from NSSA ABR

In a topology like R1 -- R2 -- R5, with R2 being NSSA ABR and R5 being
ASBR redistributing external routes, the ABR R2 will translate type-7
LSA into type-5 and advertise to the backbone. In the current implementation
R2 is also advertising a type-4 LSA when there is no need.

RFC 3101: "...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..."

With this PR a type-4 LSA will not be advertised

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