LU-2625 scripts: prevent several errors in lustre_rmmod

This fix prevents lustre_rmmod scripts from returning
errors when lustre modules are partially or not loaded.

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: Iefb4166b9b14315319858031f53ecb148c589474
Reviewed-on: http://review.whamcloud.com/5040
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>