x86: CPA use the existing pfn in split as well

When splitting large pages, we ge the pfn from the existing entry
instead of calculating it ourself.

This removes the last remaining range restriction of the cpa code.

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