LU-2914 lfsck: split LFSCK code from mdd to lfsck

The new LFSCK is used not only for namespace consistency verification,
but also for some other purposes, such as file layout consistency, DNE
consistency, and ect.

So it is more reasonable to move the new LFSCK code from mdd module to
another independent module -- lfsck, which can directly talk with lod,
local storage lib and ldlm. MDD (and OFD in next pahse) will register
as lfsck instance for related lfsck component(s), such as 'namespace'.
The lfsck processing will be independent from MDD, except the control:
start/stop/pause.

Test-Parameters: testlist=sanity-scrub,sanity-lfsck

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ia6cd619d8972f4a51effa1fe48099fced6875ab9
Reviewed-on: http://review.whamcloud.com/6321
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>