x86: mask NX from pte_pfn

In 32-bit PAE, mask NX from pte_pfn, since it isn't part of the PFN.
This code is due for unification anyway, but this fixes a latent bug.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>