osd: always update up_thru if pg changes before going active

osd: always update up_thru if pg changes before going active

We already required this if prior PG members were down, so this
affected the 'failure' case.  We now also require it for
non-failure PG changes (expansion, migration).

This fixes our maybe_went_rw calculation for prior PG intervals,
which is based on up_thru.  If maybe_went_rw is false when the
pg actually went rw, we can lose (and have lost) data.  But it is
not practical to calculate without up_thru being consistently
updated, because determining whether a pg would have been able to
go active depends on knowing last_epoch_started at a previous
point in time, which then determines how many prior intervals
may have been considered, which in turn determines whether
up_thru would have been updated, etc.  Much simpler to update it
all the time.

This should not impose a significantly greater cost, since we
already need it for the failure case.  And in general the
migration/expansion/whatever case is no more common nor critical
than the failure case.