LU-6142 lustre: remove module_vars arg to class_register_type()

LU-6142 lustre: remove module_vars arg to class_register_type()

The module_vars arg to class_register_type() is always NULL.  So it
can be removed.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ie8d7e79075ba068dec606cc9dfcc38a90e371e5b
Reviewed-on: https://review.whamcloud.com/39383
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>