rcutorture: Add ability to limit callback-flood intensity

The RCU tasks flavors of RCU now need concurrent callback flooding to
test their ability to switch between single-queue mode and per-CPU queue
mode, but their lack of heavy-duty forward-progress features rules out
the use of rcutorture's current callback-flooding code.  This commit
therefore provides the ability to limit the intensity of the callback
floods using a new ->cbflood_max field in the rcu_operations structure.
When this field is zero, there is no limit, otherwise, each callback-flood
kthread allocates at most ->cbflood_max callbacks.

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