LU-3118 lfsck: resume LFSCK from the last checkpoint

LU-3118 lfsck: resume LFSCK from the last checkpoint

It is a misc patch to enable the support to resume LFSCK from the last
checkpoint: no object will be skipped, not repeatly scan the object(s)
in front of checkpoint.

Other fixes:
1) Simplify LFSCK checkpoint logic.
2) Add linkEA for .lustre itself, then LFSCK will not be misguided.
3) Set LFSCK status as "failed" if hit error at prepare phase.
4) osd_otable_it_store() should return the cursor position, instead
of the pre-load position.
5) Other code cleanup.

Test-Parameters: testlist=sanity-scrub,sanity-lfsck
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I3f6e988da323239ff2655ad4b13eae711e892ebe
Reviewed-on: http://review.whamcloud.com/6078
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>