staging: lustre: obdclass: Remove a attribute group from a kobject

All attribute group created during class_procfs_init() should be
removed.
if class_procfs_init() will fail and also in class_procfs_clean().

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>