LU-2747 mdt: use of mdt_XXX helpers

LU-2747 mdt: use of mdt_XXX helpers

replace explicit use of some obd fields by mdt helpers:
mdt_obd_name() mdt2obd_dev() mdt_lu_site() mdt_seq_site()

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: I1a67145478a143bf6444c9a248c5cc213dc91687
Reviewed-on: http://review.whamcloud.com/5261
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>