LU-3336 lfsck: recreate the lost MDT-object

LU-3336 lfsck: recreate the lost MDT-object

If the MDT-object does not exist but related OST-obejct still
alive, then when handle the orphan OST-object, the MDT-object
will be re-created under /.lustre/lost+found/MDTxxxx with the
given OST-object stripe and owner information. The other part
of the MDT-obejct layout EA will be empty which may be filled
by other orphan OST-object(s) as the LFSCK processing.

The name for the above re-created MDT-object will be named as
${MOUNT_POINT}/.lustre/lost+found/MDTxxxx/N-${FID}, the "FID"
is the MDT-object's fid. If the OST-object has zero PFID (for
some reason), then LFSCK will assign new FID to the parent.

If the MDT-object exists, but related layout EA slot is occupied
by other OST-object which may be created by former LFSCK running
for repairing dangling referenced MDT-object or may be not.

For the latter case, the LFSCK will create a new MDT-object with
the given OST-object under /.lustre/lost+found/MDTxxxx with name
"C-${FID1}-${FID2}-${idx}":
FID1: the new MDT-object's fid.
FID2: the original MDT-object's fid.
idx: index in the layout EA.

For the former one, then means the former LFSCK guess was wrong.
If such new created OST-object has never been modified which can
be indicated with lu_attr::la_ctime is zero and lu_attr::la_mode
has no "S_ISUID" set, then the LFSCK will destroy it and replace
it with the orphan OST-object; otherwise the LFSCK will keep the
new data, and create a new MDT-object to reference the orphan as
describe above with name "C-${FID1}-${FID2}-${idx}".

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ia7700f7b2e4e09d6b576d025295fdb5c10ca90e0
Reviewed-on: http://review.whamcloud.com/7811
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>