x86: use pteval_t for _PAGE_FOO

Rather than making _PAGE_* constants signed, and then relying on
sign-extension to make sure that masks derived from them are wide
enough, just explicitly type them pteval_t.  This guarantees that they
and any derived values are the right size for the current pte format.

The reliance on sign extension is fragile, and invokes some very
subtle corners of the C type system.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>