rcutorture: Correct name of use_softirq module parameter

rcutorture: Correct name of use_softirq module parameter

BugLink: https://bugs.launchpad.net/bugs/2034469
[ Upstream commit b409afe0268faeb77267f028ea85f2d93438fced ]

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: e2b949d54392 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>