LU-5731 osp: flush async updates for osp_sync

LU-5731 osp: flush async updates for osp_sync

Current osp_sync() only considers the async requests that are
handled by the osp_sync_thread, but ignores the async updates
that are handled directly by the background ptlrpcd threads.
Usually, such async updates are for LFSCK remote repairing.
This patch will flush all of them when dt_sync() is called.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I0e6d54120acbd8ab82cf776222277ae3b805812d
Reviewed-on: http://review.whamcloud.com/12359
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>