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>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Author: paulmckrcu

tools/testing/selftests/rcutorture/configs/rcu/TREE03.boot

@@ -5,3 +5,4 @@ rcutree.gp_cleanup_delay=3
 rcutree.kthread_prio=2
 threadirqs
 rcutree.use_softirq=0
+rcutorture.preempt_duration=10