xen: fix truncation of machine address

arbitrary_virt_to_machine can truncate a machine address if its above
4G.  Cast the problem away.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/mmu.c

@@ -186,7 +186,7 @@ xmaddr_t arbitrary_virt_to_machine(unsigned long address)
 
 	BUG_ON(pte == NULL);
 
-	return XMADDR((pte_mfn(*pte) << PAGE_SHIFT) + offset);
+	return XMADDR(((phys_addr_t)pte_mfn(*pte) << PAGE_SHIFT) + offset);
 }
 
 void make_lowmem_page_readonly(void *vaddr)