rcutorture: Make the TREE03 scenario do preemption

This commit adds the rcutorture.preempt_duration module parameter to
rcutorture's TREE03.boot parameter list in order to better test preemption
of RCU read-side critical sections.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE03.boot b/tools/testing/selftests/rcutorture/configs/rcu/TREE03.boot
index 8e50bfd4..d0e8244 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE03.boot
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE03.boot
@@ -5,3 +5,4 @@
 rcutree.kthread_prio=2
 threadirqs
 rcutree.use_softirq=0
+rcutorture.preempt_duration=100