rcutorture: Cause TREE02 and TREE10 scenarios to do more callback flooding

This commit enables two callback-flood kthreads for the TREE02 scenario
and 28 for the TREE10 scenario.

Cc: Neeraj Upadhyay <neeraj.iitr10@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>