rcutorture: Make TREE03 use real-time tree.use_softirq setting

TREE03 tests RCU priority boosting, which is a real-time feature.
It would also be good if it tested something closer to what is
actually used by the real-time folks.  This commit therefore adds
tree.use_softirq=0 to the TREE03 kernel boot parameters in TREE03.boot.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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

@@ -4,3 +4,4 @@ rcutree.gp_init_delay=3
 rcutree.gp_cleanup_delay=3
 rcutree.kthread_prio=2
 threadirqs
+tree.use_softirq=0