LU-3469 osp: osp_sync() to flush pending changes synchronously

also a test added to sanity to verify this.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Iab153ec5ff1e9031a721530c5eee10a1f01adf7a
Reviewed-on: http://review.whamcloud.com/10046
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>