ospf6d: ECMP for external routes

ospf6d: ECMP for external routes

The route being added check its origin matches,
with any of the existing path (list of paths).
Remove the existing path, add if its cost is
eqaual or less than any of the existing path.

For a given route and of existing path cost is lower
(better) than one being added, discard new route update.

The existing path cost is higher (lower) than one being
added, ospf6_route_add replaces existing with new route
info.

Compare cost between delete request and with existing
route.

Ticket:CM-16139

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