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>