rcu: Use CONFIG_PREEMPTION where appropriate

The config option `CONFIG_PREEMPT' is used for the preemption model
"Low-Latency Desktop". The config option `CONFIG_PREEMPTION' is enabled
when kernel preemption is enabled which is true for the preemption model
`CONFIG_PREEMPT' and `CONFIG_PREEMPT_RT'.

Use `CONFIG_PREEMPTION' if it applies to both preemption models and not
just to `CONFIG_PREEMPT'.

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: Davidlohr Bueso <dave@stgolabs.net>
Cc: rcu@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1694a6b..c9dbb05 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2698,9 +2698,9 @@ EXPORT_SYMBOL_GPL(kfree_call_rcu);
 
 /*
  * During early boot, any blocking grace-period wait automatically
- * implies a grace period.  Later on, this is never the case for PREEMPT.
+ * implies a grace period.  Later on, this is never the case for PREEMPTION.
  *
- * Howevr, because a context switch is a grace period for !PREEMPT, any
+ * Howevr, because a context switch is a grace period for !PREEMPTION, any
  * blocking grace-period wait automatically implies a grace period if
  * there is only one CPU online at any point time during execution of
  * either synchronize_rcu() or synchronize_rcu_expedited().  It is OK to