LU-4970 lfsck: flush async updating before exit

LU-4970 lfsck: flush async updating before exit

Otherwise, the test scripts may get some internal status by race.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I4af07cf91f6b6c77d5cab67fc0df21b27174ee4c
Reviewed-on: http://review.whamcloud.com/11276
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Andreas Dilger <andreas.dilger@intel.com>