LU-4134 obdclass: fix double free in failure path

LU-4134 obdclass: fix double free in failure path

We should just decref for obd if register is failed.
Since class_export_put will invoke class_free_dev,
then we may face a double free.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: Ia8d1e487c69c4de1c7c247158cc8615aa6b6093a
Reviewed-on: https://review.whamcloud.com/29967
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>