x86/xen: fix arbitrary_virt_to_machine()

While I realize that the function isn't currently being used, I still
think an obvious mistake like this should be corrected.

Signed-off-by: Jan Beulich <[e-mail]>
Acked-by: Jeremy Fitzhardinge <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>

arch/x86/xen/mmu.c

···
 {
 	unsigned int level;
 	pte_t *pte = lookup_address(address, &level);
-	unsigned offset = address & PAGE_MASK;
+	unsigned offset = address & ~PAGE_MASK;
 
 	BUG_ON(pte == NULL);