LU-10565 osd: use readdir while iterate is kabi_extend

LU-10565 osd: use readdir while iterate is kabi_extend

Sometime iterate interface is not initialized in ldiskfs.
So we use readdir in such case.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: I43260a6d27003895b0ddd1bdf7b0539cc2ad64c5
Reviewed-on: https://review.whamcloud.com/31018
Tested-by: Jenkins
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>