LU-1729: lu_buf code cleaning

Fix DLUBUF define and use LU_BUF_NULL to clear a lu_buf

Change-Id: I742308616d9c39196e56bf4983523152d26e1245
Signed-off-by: jcl <jacques-charles.lafoucriere@cea.fr>
Reviewed-on: http://review.whamcloud.com/3589


Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>