scftorture: Dump task cpumask when running on the wrong CPU

The scftorture_invoker() function complains if a given kthread is running
on the wrong CPU, but does not print the offending kthread's cpumask.
This commit therefore adds current->cpus-ptr to the WARN_ONCE() output.

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