staging/lustre/llapi: add user space method for lov_user_md

move lov_mds_md_size from obd_lov.h to lustre_idl.h
to have it close to lov_mds_md definition.

add lov_user_md_size() to compute lum size so
llapi and user space utils do not use kernel internal
definitions/methods

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3345
Lustre-change: http://review.whamcloud.com/6345


Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>