x86: mtrr_cleanup optimization, v2

fix hpa's t61 with 4g ram:

   change layout from
	(n - 1)*chunksize + chunk_size - NC
   to
	n*chunksize - NC

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>