ospf6d: fix infinite loop when adding ASBR route

ospf6d: fix infinite loop when adding ASBR route

Commit 8f359e1593c414322 removed a check that prevented the same route
from being added twice. In certain topologies, that change resulted in
the following infinite loop when adding an ASBR route:

ospf6_route_add
 ospf6_top_brouter_hook_add
  ospf6_abr_examin_brouter
   ospf6_abr_examin_summary
    ospf6_route_add
     (repeat until stack overflow)

Revert the offending commit and update `ospf6_route_is_identical()` to
not do comparison using `memcmp()`.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>