x86: cpa, preserve large pages if possible

When CPA is called on a range which fits into a large page mapping,
avoid to split the page when:

1) There is no change of attributes
2) The range to change is a complete large mapping

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>