LU-2915 lfsck: LFSCK 1.5 technical debts (2)

LU-2915 lfsck: LFSCK 1.5 technical debts (2)

This patch resolves some LFSCK 1.5 technical debts, including:
1) LFSCK does NOT hold spin_lock when call dt iteration API(s).
2) Remove the hack of using dt_it_ops::put() method for events
   notification to low layer otable-based iteration.

Test-Parameters: testlist=sanity-scrub,sanity-lfsck
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ie16161370ac218860a74b23d853c4c378adce4ec
Reviewed-on: http://review.whamcloud.com/6343
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>