rcutorture: Test RCU Tasks lock-contention detection

This commit adjusts the TRACE02 scenario to use a pair of callback-flood
kthreads.  This in turn forces lock contention on the single RCU Tasks
Trace callback queue, which forces use of all CPUs' queues, thus testing
this transition.  (No, there is not yet any way to transition back.

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