xen64: get active_mm from the pda

x86_64 stores the active_mm in the pda, so fetch it from there.
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

@@ -805,8 +805,15 @@ void xen_dup_mmap(struct mm_struct *oldmm, struct mm_struct *mm)
 static void drop_other_mm_ref(void *info)
 {
 	struct mm_struct *mm = info;
+	struct mm_struct *active_mm;
 
-	if (__get_cpu_var(cpu_tlbstate).active_mm == mm)
+#ifdef CONFIG_X86_64
+	active_mm = read_pda(active_mm);
+#else
+	active_mm = __get_cpu_var(cpu_tlbstate).active_mm;
+#endif
+
+	if (active_mm == mm)
 		leave_mm(smp_processor_id());
 
 	/* If this cpu still has a stale cr3 reference, then make sure