rcutorture: Test extended "rcu" read-side critical sections

This commit makes the "rcu" torture type test extended read-side
critical sections in order to test the deferral of RCU-preempt
quiescent-state testing.

In CONFIG_PREEMPT=n kernels, this simply duplicates the setup already
in place for the "sched" torture type.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcutorture.c

@@ -431,6 +431,7 @@ static struct rcu_torture_ops rcu_ops = {
 	.stats		= NULL,
 	.irq_capable	= 1,
 	.can_boost	= rcu_can_boost(),
+	.extendables	= RCUTORTURE_MAX_EXTEND,
 	.name		= "rcu"
 };