xen: mask _PAGE_PCD from ptes

_PAGE_PCD maps a page with caching disabled, which is typically used for
mapping harware registers.  Xen never allows it to be set on a mapping, and
unprivileged guests never need it since they can't see the real underlying
hardware.  However, some uncached mappings are made early when probing the
(non-existent) APIC, and its OK to mask off the PCD flag in these cases.

This became necessary because Xen started checking for this bit, rather
than silently masking it off.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>