OSD: OSDMapRef access by multiple threads is unsafe

OSD: OSDMapRef access by multiple threads is unsafe

we update OSD::osdmap in OSD::_committed_osd_maps() which is executed
by objectstore's finisher thread. while PG::sched_scrub() is called
by OSD's sharded work queue's worker thread.and we push the osdmap
updates down to PGs OSD::consume_map() which is in turn called by
OSD::_committed_osd_maps() where osdmap is updated. so it does not big
deal if we are checking a stale CEPH_OSDMAP_NODEEP_SCRUB flag.
also this flag will be updated with the latest osdmap very soon.

Signed-off-by: Kefu Chai <kchai@redhat.com>
Signed-off-by: Zengran Zhang <zhangzengran@sangfor.com.cn>