LU-2441 mdd: Implement volatile file

Add a way to create files which are created unlinked, so
they are automatically removed at last close

User interface is:
- llapi_create_volatile_idx()
- llapi_create_volatile()

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: Ia33a6832d31b83191e4d364da4350cedbe6eef39
Reviewed-on: http://review.whamcloud.com/4836
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>