LU-1866 lfsck: LFSCK for namespace consistency (1)

LU-1866 lfsck: LFSCK for namespace consistency (1)

It is the first part of LFSCK 1.5 for namespace consistency
check and repair. It mainly implements the LFSCK namespace
component internal framework and external APIs.

The functionality parts for the LFSCK namespace component,
such as lfsck_operations::lfsck_{exec_dir,dump,double_scan},
are not included. They will be implemented in other patches.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I011e06c991577e632eadf364b87be74f1f5535c6
Reviewed-on: http://review.whamcloud.com/4910
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
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>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>