LU-2915 doc: update Lustre doc for LFSCK

Add section to describe the new LFSCK.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I11f1763a1f1527f8a7c0893df0fbce2788679775
Reviewed-on: http://review.whamcloud.com/5913
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Hudson
Reviewed-by: Richard Henwood <richard.henwood@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>