LU-2378 lma: move HSM & SOM attributes to dedicated xattrs

LU-2378 lma: move HSM & SOM attributes to dedicated xattrs

Reduce LMA to its minimum and move HSM & SOM attributes to dedicated
on-disk extented attributes. This is more consistent with the MDS
layering (HSM, LMA & SOM are managed in different layers) and avoids
locking + read/modify/write in many cases.

As far as interoperability is concerned, the old code works just fine
if it reads a smaller LMA size and the new code retries with a
larger buffer on ERANGE error.

Signed-off-by: Johann Lombardi <johann.lombardi@intel.com>
Change-Id: Id4ee465ea69820229f7214d800821864330a8aec
Reviewed-on: http://review.whamcloud.com/4746
Reviewed-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>