LU-3373 osd-ldiskfs: readdir replace by iterate

LU-3373 osd-ldiskfs: readdir replace by iterate

Use iterate instead of readdir callback in iop.

Signed-off-by: yang sheng <yang.sheng@intel.com>
Change-Id: Icb08292009c965ca693814e854ae2e77b3e7a4f0
Reviewed-on: http://review.whamcloud.com/8231
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Peng Tao <bergwolf@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>