drm/i915/buddy: tidy up i915_buddy_fini

If we are leaking nodes don't hide it. Also stop trying to be
"defensive" and instead embrace Kasan et al.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190816105357.14340-1-matthew.auld@intel.com