ospfd: NSSA area should not exchange Type-4 LSAs in DD

ospfd: NSSA area should not exchange Type-4 LSAs in DD

Problem Statement:
==================
OSPF Peer gets stuck in EXSTART with ARISTA Device.

Root Cause:
=================
First peer is form with Arista device in normal area and then
the area type is changed to NSSA. Due to this Type-4 and Type-5
LSAs advertised by Arista router is still present in
the OSPF DB. While DD exchange the Type-5 LSAs are omitted but
the Type-4 LSAs are not omitted due to which Arista device gets
stuck in EXSTART and it keeps moving between EXCHANGE And EXSTART.

Fix:
=================
When the area is NSSA, we should not send Type-4 LSAs in DD
exchange packet.

Signed-off-by: Mobashshera Rasool <mrasool@vmware.com>