rcu: Silence unused-variable warnings

The "idle" variable in both rcu_eqs_enter_common() and
rcu_eqs_exit_common() is only used in a WARN_ON_ONCE().  If the kernel
is built disabling WARN_ON_ONCE(), the compiler will complain (rightly)
that "idle" is unused.  This commit therefore adds a __maybe_unused to
the declaration of both variables.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree.c

@@ -371,7 +371,8 @@ static void rcu_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval,
 {
 	trace_rcu_dyntick(TPS("Start"), oldval, rdtp->dynticks_nesting);
 	if (!user && !is_idle_task(current)) {
-		struct task_struct *idle = idle_task(smp_processor_id());
+		struct task_struct *idle __maybe_unused =
+			idle_task(smp_processor_id());
 
 		trace_rcu_dyntick(TPS("Error on entry: not idle task"), oldval, 0);
 		ftrace_dump(DUMP_ORIG);
@@ -508,7 +509,8 @@ static void rcu_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval,
 	rcu_cleanup_after_idle(smp_processor_id());
 	trace_rcu_dyntick(TPS("End"), oldval, rdtp->dynticks_nesting);
 	if (!user && !is_idle_task(current)) {
-		struct task_struct *idle = idle_task(smp_processor_id());
+		struct task_struct *idle __maybe_unused =
+			idle_task(smp_processor_id());
 
 		trace_rcu_dyntick(TPS("Error on exit: not idle task"),
 				  oldval, rdtp->dynticks_nesting);