ospf6d: fix area border router duplicate

ospf6d: fix area border router duplicate

Avoid duplicate ABR brouter entry and
its nexthops.
The route lookup results in first element of the
route/redix node. In case of Intra and inter area
brouter, the first element always intra brouter.
the first element comparison results in always addition
of new element for inter area brouter in brouter table.
Now, iterate all elements of the route node and compare
for brouter origin, if it is same simply update its
nexthops to FIB.

brouter and brouter route nexthops should be merge to avoid
duplicate nexthops for the inter area routes.

Ticket:CM-20807

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