LU-3336 lfsck: orphan OST-objects iteration

LU-3336 lfsck: orphan OST-objects iteration

During the second stage scanning, the LFSCK on the MDT(s) will scan
the orphan OST-objects via OSP level iteration which fetches remote
orphan OST-objects information via OBD_IDX_READ RPC, and shares the
existing framework/functions with others, such as quota.

Implement the sponsor (the master LFSCK engine on the MDT) logic
for the orphan OST-objects iteration.

Implement LFSCK layout rbtree iteration - lfsck_orphan_index_ops,
for slave LFSCK on OST. The lfsck_orphan_index_ops is registered
onto the rbtree object. The incoming OBD_IDX_READ RPC for orphan
OST-object scanning will iterate the rbtree via dt_index_read to
call the registered lfsck_orphan_index_ops.

Others:
1) Speed control during the second-phase scanning.
2) The LFSCK layout trace file (on the MDT) flags should be set
   with LF_INCOMPLETE if LFSCK slave on OST restart or failed.
3) Some code cleanup.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I67d5d870dbf9b80530f4d61ed1a3e5b5df70b1a0
Reviewed-on: http://review.whamcloud.com/8303
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>