x86,xen: use current->state helpers

Call __set_current_state() instead of assigning the new state directly.
These interfaces also aid CONFIG_DEBUG_ATOMIC_SLEEP environments,
keeping track of who changed the state.
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/smp.c

@@ -507,7 +507,7 @@ static int xen_cpu_disable(void)
 static void xen_cpu_die(unsigned int cpu)
 {
 	while (xen_pv_domain() && HYPERVISOR_vcpu_op(VCPUOP_is_up, cpu, NULL)) {
-		current->state = TASK_UNINTERRUPTIBLE;
+		__set_current_state(TASK_UNINTERRUPTIBLE);
 		schedule_timeout(HZ/10);
 	}