LU-1267 lfsck: framework (1) for MDT-OST consistency

LU-1267 lfsck: framework (1) for MDT-OST consistency

1) New LFSCK component - lfsck_layout, its data structure, tracing
   file, lFSCK APIs, internal shared functions, and so on.

2) Extend the existing LFSCK:
2.1) new flag - LF_INCOMPLETE, for the case of some target (MDT/OST)
     not join the LFSCK or crashed during the LFSCK.
2.2) New status - LS_PARTIAL, corresponding to above LF_INCOMPLETE,
     if some target (MDT/OST) does not join the LFSCK processing or
     crashed during the LFSCK, when it finished, its status will be
     set as LS_PARTIAL.

3) Control the LFSCK speed by each component itself during the
   second-phase scanning.

4) Avoid lfsck engines to access freed lfsck_instance.

5) Some code cleanup.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I6dc7bb881dc831f6c760be14aac2a066ad75ffec
Reviewed-on: http://review.whamcloud.com/7146
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>