rcu: Replace deprecated CPU-hotplug functions

The functions get_online_cpus() and put_online_cpus() have been
deprecated during the CPU hotplug rework. They map directly to
cpus_read_lock() and cpus_read_unlock().

Replace deprecated CPU-hotplug functions with the official version.
The behavior remains unchanged.

Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: rcu@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -4011,7 +4011,7 @@ void rcu_barrier(void)
 	 */
 	init_completion(&rcu_state.barrier_completion);
 	atomic_set(&rcu_state.barrier_cpu_count, 2);
-	get_online_cpus();
+	cpus_read_lock();
 
 	/*
 	 * Force each CPU with callbacks to register a new callback.
@@ -4042,7 +4042,7 @@ void rcu_barrier(void)
 					  rcu_state.barrier_sequence);
 		}
 	}
-	put_online_cpus();
+	cpus_read_unlock();
 
 	/*
 	 * Now that we have an rcu_barrier_callback() callback on each