drm/buddy: Add a testcase to verify the multiroot fini

- Added a testcase to verify the multiroot force merge fini.
- Added a new field in_use to track the mm freed status.

v2:(Matthew)
  - Add kunit_fail_current_test() when WARN_ON is true.

Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Signed-off-by: Lin.Cao <lincao12@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241226070116.309290-2-Arunpravin.PaneerSelvam@amd.com