LU-4075 osd: handle getxattr for trusted.version

LU-4075 osd: handle getxattr for trusted.version

In the ldiskfs osd_xattr_get() remove an assertion on the size of the
supplied buffer and add normal handling.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ie4d3b4ab6dd1e8328404d1f8e5b403130b62e64a
Reviewed-on: http://review.whamcloud.com/11649
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>