LU-5848 debug: more debug log for dt_sync

Add some D_CACHE logs at the entry/exit for osp_sync()/osd_sync().

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Iaa7fbfbbadb9312528b5092d64615b277de6b679
Reviewed-on: http://review.whamcloud.com/12573
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>