LU-3345 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

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: I01825efb4521bf90293239054a32fd66f85bcaf2
Reviewed-on: http://review.whamcloud.com/6345
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>