LU-2214 lod: fix tricky iterator methods

LU-2214 lod: fix tricky iterator methods

instead of bypassing LOD layer in the iterator methods,
just get own iterator structure in lod, which keep references
to the object and the iterator of the layer below.

this also let LOD to have different iterators in different
objects which is required for DNE.

to verify the approach lfsck goes through LOD now.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I62935319a686f4b06b2cdf5ea4002a800c0c430d
Reviewed-on: http://review.whamcloud.com/4370
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: wangdi <di.wang@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>