ospfd: Make external routes in ospf VRF aware

ospfd: Make external routes in ospf VRF aware

Currently, ospf external routers are part of struct
ospf_master which is not vrf aware ospf instance.
All ospf external routes are injected/leaked into all
vrfs.

Moved ospf external routes db to struct ospf to make
vrf aware, such one external routes learnt in one vrf
is not leaked into another vrf.

Ticket:CM-18855
Testing Done:
Inject external route in non-default vrf x, validated
ospf database across the vrf x, validated ospf routes
for vrf x.

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