powerpc: avoid lockdep when we are offline

This is second version of my fix to PPC's  "WARNING: suspicious RCU usage",
I improved my fix under Paul E. McKenney's guidance:
Link: https://lore.kernel.org/lkml/[email protected]/T/

During the cpu offlining, the sub functions of xive_teardown_cpu will
call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will
travel RCU protected list, so "WARNING: suspicious RCU usage" will be
triggered.

Avoid lockdep when we are offline.

Signed-off-by: Zhouyi Zhou <[email protected]>

arch/powerpc/platforms/pseries/hotplug-cpu.c

@@ -64,10 +64,15 @@ static void pseries_cpu_offline_self(void)
 
 	local_irq_disable();
 	idle_task_exit();
+	/* prevent lockdep code from traveling RCU protected list
+	 * when we are offline.
+	 */
+	lockdep_off();
 	if (xive_enabled())
 		xive_teardown_cpu();
 	else
 		xics_teardown_cpu();
+	lockdep_on();
 
 	unregister_slb_shadow(hwcpu);
 	rtas_stop_self();