osd/PG: create new PGs from activate in last_peering_reset epoch

osd/PG: create new PGs from activate in last_peering_reset epoch

If we create a new PG (e.g., a backfill target) in the current epoch, it
might be > last_peering_reset.  That can lead to last_peering_reset on
the replica having a higher last_peering_reset than the primary's, which
can then lead to future messages, like pg_scan during backfill, being
ignored.

Fixes: http://tracker.ceph.com/issues/24452
Signed-off-by: Sage Weil <sage@redhat.com>