scftorture: Avoid false-positive warnings in scftorture_invoker()

If the call to set_cpus_allowed_ptr() in scftorture_invoker()
fails, a later WARN_ONCE() complains.  But with the advent of
570a752b ("lib/smp_processor_id: Use is_percpu_thread() instead of
nr_cpus_allowed"), this complaint can be drowned out by complaints from
smp_processor_id().  The rationale for this change is that scftorture's
kthreads are not marked with PF_NO_SETAFFINITY, which means that a system
administrator could change affinity at any time.

However, scftorture is a torture test, and the system administrator might
well have a valid test-the-test reason for changing affinity.  This commit
therefore changes to raw_smp_processor_id() in order to avoid the noise,
and also adds a WARN_ON_ONCE() to the call to set_cpus_allowed_ptr() in
order to directly detect immediate failure.  There is no WARN_ON_ONCE()
within the test loop, allowing human-reflex-based affinity resetting,
if desired.

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