ospf6d: ECMP for Intra Area Prefix routes

ospf6d: ECMP for Intra Area Prefix routes

Handle ECMP for Intra Area Prefix LSAs routes.

Ticket:CM-16139
Testing Done:

Configure ospf6 passive interface R2 and R3 with
same prefix address.
Check Intra Area Prefix LSA update  at R1 and R3
which would have ECMP paths with effective two
paths and two nexthops (from R2 and R4).
stop frr at R3 and R1 and R4 route changes back to
one nexthop and one paht.
R1 ---- R2
|       |
R3 ---- R4

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