rcutorture: Correct name of use_softirq module parameter

The BUSTED-BOOST and TREE03 scenarios specify a mythical tree.use_softirq
module parameter, which means a failure to get full test coverage.  This
commit therefore corrects the name to rcutree.use_softirq.

Fixes: e2b949d5 ("rcutorture: Make TREE03 use real-time tree.use_softirq setting")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

tools/testing/selftests/rcutorture/configs/rcu/BUSTED-BOOST.boot

@@ -5,4 +5,4 @@ rcutree.gp_init_delay=3
 rcutree.gp_cleanup_delay=3
 rcutree.kthread_prio=2
 threadirqs
-tree.use_softirq=0
+rcutree.use_softirq=0

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

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