arm64, debug-monitors: Fix CPU hotplug callback registration

Subsystems that want to register CPU hotplug callbacks, as well as perform
initialization for the CPUs that are already online, often do it as shown
below:

	get_online_cpus();

	for_each_online_cpu(cpu)
		init_cpu(cpu);

	register_cpu_notifier(&foobar_cpu_notifier);

	put_online_cpus();

This is wrong, since it is prone to ABBA deadlocks involving the
cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
with CPU hotplug operations).

Instead, the correct and race-free way of performing the callback
registration is:

	cpu_notifier_register_begin();

	for_each_online_cpu(cpu)
		init_cpu(cpu);

	/* Note the use of the double underscored version of the API */
	__register_cpu_notifier(&foobar_cpu_notifier);

	cpu_notifier_register_done();

Fix the debug-monitors code in arm64 by using this latter form of callback
registration.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Ingo Molnar <mingo@kernel.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

arch/arm64/kernel/debug-monitors.c

@@ -155,12 +155,16 @@ static struct notifier_block os_lock_nb = {
 
 static int debug_monitors_init(void)
 {
+	cpu_notifier_register_begin();
+
 	/* Clear the OS lock. */
 	smp_call_function(clear_os_lock, NULL, 1);
 	clear_os_lock(NULL);
 
 	/* Register hotplug handler. */
-	register_cpu_notifier(&os_lock_nb);
+	__register_cpu_notifier(&os_lock_nb);
+
+	cpu_notifier_register_done();
 	return 0;
 }
 postcore_initcall(debug_monitors_init);