ospf6d: retain inter area border router type-4

ospf6d: retain inter area border router type-4

During Intra area border router calculation, all
border routers are  marked for remove from brouter table.
Once SPF calculation is done, retain inter area
border router if the adv. intra border router (abr)
is present in SPF table.

Ticket:CM-20171
Testing Done:
Validated inter area ASBR (L1) is retained at
R1 and R2 post intra border router calculation.

L1 -- (area 1)-- L2 -- (area 0) -- R1 --- R2

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