PeeringState: don't zero backfill target num_bytes on activation

PeeringState: don't zero backfill target num_bytes on activation

834d3c19a774f1cc93903447d91d182776e12d18 preserves num_bytes
on backfill targets in order to estimate space required to complete
backill.  However, from activation until backfill reservation,
info.stats.stats.sum.num_bytes is persisted to disk as 0 messing
up future intervals.  Instead, preserve it in the info sent during
recovery and leave it alone in RequestBackfillPrio.

Additionally, it's possible for backfill to be preempted between
last_backfill=MAX being sent to the replica and Backfilled being
queued occuring.  In that case, the stats get on reservation
and the replica ends up with invalid stats.

Fixes: https://tracker.ceph.com/issues/39401
Signed-off-by: sjust@redhat.com <sjust@redhat.com>