LU-957 scrub: osd-ldiskfs itable based iteration

LU-957 scrub: osd-ldiskfs itable based iteration

Implement inode table based object iteration in osd-ldiskfs.
It is implemented as DT iteration APIs, which can be used by
up layer LFSCK to scan the whole device sequentially.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I3d80bb4a174d47429764e5cca35e4f07be52d50b
Reviewed-on: http://review.whamcloud.com/2553
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>