rcutorture: Add missing destroy_timer_on_stack()

The rcu_torture_reader() function uses an on-stack timer_list structure
which it initializes with setup_timer_on_stack().  However, it fails to
use destroy_timer_on_stack() before exiting, which results in leaking a
tracking object if DEBUG_OBJECTS is enabled.  This commit therefore
invokes destroy_timer_on_stack() to avoid this leakage.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcutorture.c

@@ -1023,8 +1023,10 @@ rcu_torture_reader(void *arg)
 		cond_resched();
 		stutter_wait("rcu_torture_reader");
 	} while (!torture_must_stop());
-	if (irqreader && cur_ops->irq_capable)
+	if (irqreader && cur_ops->irq_capable) {
 		del_timer_sync(&t);
+		destroy_timer_on_stack(&t);
+	}
 	torture_kthread_stopping("rcu_torture_reader");
 	return 0;
 }