selftests/resctrl: Unmount resctrl FS if child fails to run benchmark

A child calls PARENT_EXIT() when it fails to run a benchmark to kill
the parent process. PARENT_EXIT() lacks unmount for the resctrl FS and
the parent won't be there to unmount it either after it gets killed.

Add the resctrl FS unmount also to PARENT_EXIT().

Fixes: 591a6e85

 ("selftests/resctrl: Add basic resctrl file system operations and data")
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Tested-by: Shaopeng Tan (Fujitsu) <tan.shaopeng@fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>