selftests/resctrl: Unmount resctrl FS before starting the first test

Resctrl FS mount/remount/umount code is hard to track. Better approach
is to use mount/umount pair for each test but that assumes resctrl FS
is not mounted beforehand.

Change umount_resctrlfs() so that it can unmount resctrl FS from any
path, and enable further simplifications into mount/remount/umount
logic by unmounting resctrl FS at the start if a pre-existing
mountpoint is found.

Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.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>