ospf6d: Show neighbor displays twice for every neighbor.

ospf6d: Show neighbor displays twice for every neighbor.

Problem Statement:
=================
When show ipv6 ospf6 neighbor is executed each neighbor gets
displayed twice.

frr# show ipv6 ospf6 neighbor
Neighbor ID     Pri    DeadTime    State/IfState         Duration I/F[State]
1.1.1.1           1    00:00:39     Full/DR              00:00:10 ens192[BDR]
1.1.1.1           1    00:00:39     Full/DR              00:00:10 ens192[BDR]

RCA:
====
Since ospf6_interface gets added twice to if_list we see multiple
display output.

Fix:
====
add ospf6_interface to if_list only when newly area gets attached
to the interface. If its already added no need to add it again.

Risk:
=====
Low

Tests Executed:
===============
Have tested the defect scenario.

frr# show ipv6 ospf6 neighbor
Neighbor ID     Pri    DeadTime    State/IfState         Duration I/F[State]
1.1.1.1           1    00:00:35     Full/DR              00:36:04 ens192[BDR]
frr#

Signed-off-by: Abhinay Ramesh <rabhinay@vmware.com>