ospf6d: assign zebra router-id to ospf6 instance

ospf6d: assign zebra router-id to ospf6 instance

Store zebra router-id in global structure.
Before router ospf6 instance created,
zebra router-id callback called.

During ospf6 main execution zebra init happens,
but default instance does not execute until
cli replay 'router ospf6'.
Call ospf6_router_id_change during 'router ospf6'
to assign zebra router id to ospf6 instance.

Ticket:CM-19937
Testing Done:
Assign Loopback /32 (6.6.6.6/32) address,
restart frr with (router ospf6 in frr.conf).
ospf6 default instance assigned 6.6.6.6 router-id.

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>