ospf6d: Fix summary LSA removal

[gromit1811]

Finally, a first attempt to fix #16197

The main fixes are from the following 2 commits (other commits are mostly cleanup/optimization):

commit 9eb0492

ospf6d: Fix old summary route lookup

When orignating summaries, we perform a lookup to find possible previous
summaries. Don't just compare the prefix in that case, but also route and
path properties: We might have several summary entries for the same
destination (e.g. with a intra- and inter-area path) at the same time (at
least temporarily) and we need to deal with the right one.

commit 6f8df0c

ospf6d: Fix summary LSA removal by using ospf6_abr_task()

Previously, we checked each individual route to see whether a summary LSA
needs to be originated or purged. The problem is that this decision can't be
made reliably by looking at a single route, because there might be several
routes for a destination (e.g. one with a intra- and one with an inter-area
path) and one might cause origination of a summary LSA while the other
causes it to be purged. This causes unstable and/or missing LSAs as
described in https://github.com/FRRouting/frr/issues/16197

Instead, ospf6_abr_originate_summary_to_area() now only originates new LSAs
or approves existing ones. Removal is handled in ospf6_abr_task() after the
whole routing table has been iterated. LSAs which have not been approved by
any call to ospf6_abr_originate_summary_to_area() are then removed. This
matches the behaviour of OSPFv2.

Note that this isn't finished/complete yet and is intended mainly for feedback or to stop me in case I'm moving in a completely wrong direction 😉

Open issues:

• Check topotest failures: ospf6_gr_topo1 ospf6_topo2
• Routing table hooks no longer make sense the way they're implemented (directly calling ospf6_abr_originate_summary_to_area()), just schedule ospf6_abr_task() instead?
• Right now we actually check routes twice, once via hooks and once via ospf6_abr_task(). Doesn't seem to hurt, but isn't really efficient.
• Check whether there are other cases where we might have to schedule ospf6_abr_task() to immediately remove a summary LSA.
• ospf6_abr_task() is hidden in ospf6_nssa.c even though it is not at all related to NSSA. Guess it was put there when adding NSSA support and that needed a small part of the ospf6_abr_task() functionality. Move non-NSSA functions from ospf6_nssa.c to ospf6_abr.c.

[donaldsharp]

Hey can we submit the PR against master not 10.2? All code must go into master first.

[gromit1811]

Sure. I'm currently testing on 10.2.1 to have a stable base, but of course I'll rebase to master when the PR is complete and before removing the draft status