xen: mask out PWT too

The hypervisor doesn't allow PCD or PWT to be set on guest ptes, so
make sure they're masked out.  Also, fix up some previous mispatching.
Signed-off-by: NJeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

arch/x86/xen/mmu.c

@@ -291,7 +291,7 @@ pte_t xen_make_pte(unsigned long pte)
 	if (pte & _PAGE_PRESENT)
 		pte = phys_to_machine(XPADDR(pte)).maddr;
 
-	pte &= ~_PAGE_PCD;
+	pte &= ~(_PAGE_PCD | _PAGE_PWT);
 
 	return (pte_t){ pte };
 }