ospfd: fix default-metric change if external LSAs already sent

ospfd: fix default-metric change if external LSAs already sent

Currently, when redistribution of routes was configured, external LSAs
were already advertised to peers, and then default-metric is changed,
external LSAs refresh will not occur. In other words, the peers will not
receive the refreshed external LSAs with the new metric.

With this fix, changing default-metric will cause external LSAs to be
refreshed and flooded.

There is a similar task to refresh external LSAs when NSSA settings are
changed. And there is a function that accomplishes it -
ospf_schedule_asbr_nssa_redist_update(). Since the function does the
general work of refreshing external LSAs and is not specific to NSSA
settings, the idea is to give it a more general name and call it when
default-metric changes in order to fix the problem.

Signed-off-by: Alexander Chernavin <achernavin@netgate.com>