xen: use PTE_MASK in pte_mfn()

Use PTE_MASK to extract mfn from pte.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Tested-by: NHugh Dickins <hugh@veritas.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/asm-x86/xen/page.h

@@ -127,7 +127,7 @@ static inline void set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 
 static inline unsigned long pte_mfn(pte_t pte)
 {
-	return (pte.pte & ~_PAGE_NX) >> PAGE_SHIFT;
+	return (pte.pte & PTE_MASK) >> PAGE_SHIFT;
 }
 
 static inline pte_t mfn_pte(unsigned long page_nr, pgprot_t pgprot)