fix the bug reported here:

	http://bugzilla.kernel.org/show_bug.cgi?id=10232

use update_memory_range() instead of add_memory_range() directly
to avoid closing the gap.

( the new code only affects and runs on systems where the MTRR
  workaround triggers. )
Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>