osd/PeeringState: do not complain about past_intervals constrained by oldest epoch

osd/PeeringState: do not complain about past_intervals constrained by oldest epoch

The start of the required interval has a floor set by the oldest osdmap
epoch we have.  That can lead to an invalid/empty required interval
(because the start is >= the end), but the PG may still have past
intervals.  That can be cause by a slow PG deletion.

No need to complain about this harmless condition.

Fixes: https://tracker.ceph.com/issues/39546
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit b1396774a40b7b3e54c9f52b6bb6f03d31c35401)

Conflicts:
        src/osd/PeeringState.cc
- this file does not exist in nautilus: manually cherry-picked the changes into
  PG.cc