ospf6d: fix intra prefix ecmp

ospf6d: fix intra prefix ecmp

When there are multiple advertisers/paths to reach
Intra-Prefix route, if any path is removed,
re-added back, it does add to the same route node.

ospf6 intra prefix, first stored in oa->route_table
then as part of add cb, it would add to ospf6->route_table
which adds to FIB.

When copying a route with its paths & NHs from oa->table to
ospf6->table the path origin should not be modified otherwise
ospf6->table would not find existing node rather it appends
new node.

Use spf_table to fetch nexthops for a given advertisers/path,
to form effective nexthop list for a route.

Ticket:CM-16139
Testing Done:
    R2 ---- R3
     |       |
    R1 ---- R4
Inject Intra Ara Prefix LSA from R1 & R3, validate R2 and R4
having two origination point/paths to reach for a route.
Trigger link flap, frr restart or remove/readd R3's INP one of the
injection point.
Remove link between R4 to R1 and validate R3 carrying single
path to reach prefix.

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