rcutorture: Allow specifying per-scenario stat_interval

The rcutorture test suite makes double use of the rcutorture.stat_interval
module parameter. As its name suggests, it controls the frequency
of statistics printing, but it also controls the rcu_torture_writer()
stall timeout. The current setting of 15 seconds works surprisingly well.
However, given that the RCU tasks stall-warning timeout is ten -minutes-,
15 seconds is too short for TASKS02, which runs a non-preemptible kernel
on a single CPU.

This commit therefore adds checks for per-scenario specification of the
rcutorture.stat_interval module parameter.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

rcutorture: Add RCU-tasks test cases

This commit adds the TASKS01 and TASKS02 Kconfig fragments, along with
the corresponding TASKS01.boot and TASKS02.boot boot-parameter files
specifying that rcutorture test RCU-tasks instead of the default flavor.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>