mon/OSDMonitor: check creating_pgs.last_scan_epoch instead when sending creates

mon/OSDMonitor: check creating_pgs.last_scan_epoch instead when sending creates

we cannot be sure that the creating_pgs_by_osd_epoch is in sync with
creating_pgs.pgs even if mapping.get_epoch() is less or equal to
creating_pgs_epoch. because, 1) the access to mapping.epoch is not
protected by lock, 2) even worse, the mapping might be not finished yet
when we are trying to send pg-creates to subscribers.

so instead of comparing creating_pgs_epoch with mapping's epoch, we
should compare it with creating_pgs.last_scan_epoch. the former is
updated once the creating_pgs_by_osd_epoch is updated with the latest
mapping's epoch and creating_pgs.pgs; the latter is updated with current
osdmap's epoch when creating_pgs is being updated with the inc osd map.
if we are using a creating_pgs_epoch in sync, creating_pgs_epoch should
be creating_pgs.last_scan_epoch + 1.

Fixes: http://tracker.ceph.com/issues/20785
Signed-off-by: Kefu Chai <kchai@redhat.com>