ospf6d: Fix flooding of old copies of self-originated LSAs

ospf6d: Fix flooding of old copies of self-originated LSAs

When receiving old copies (e.g. originated before the local ospf6d was
restarted) of supposedly self-originated LSAs which we previously tried to
flush from the network (by setting them to MaxAge), neither flood them nor
add them to our LSDB. Instead, keep the MaxAge version until we actually
(re-)originate them.

Possible fix for #7030. Testcase in #7168
(tests/topotests/ospf6-dr-no-netlsa-bug7030).

Signed-off-by: Martin Buck <mb-tmp-tvguho.pbz@gromit.dyndns.org>