OSD: avoid FileStore finisher deadlock in osd_lock when shutdown OSD

OSD: avoid FileStore finisher deadlock in osd_lock when shutdown OSD

When OSD receive signal to shutdown, it will try to call ObjectStore->flush
to make sure all pending IO done. But previous OSDMap committing IO may
complete Context in OSD::_committed_osd_maps which need to acquire osd_lock
firstly. So OSD shutdown thread need to wait for finisher done, and finisher
is acquiring osd_lock. dead lock happended

Signed-off-by: Haomai Wang <haomai@xsky.com>