kthread: Unify kthread_create_on_cpu() and kthread_create_worker_on_cpu() automatic format

kthread_create_on_cpu() uses the CPU argument as an implicit and unique
printf argument to add to the format whereas
kthread_create_worker_on_cpu() still relies on explicitly passing the
printf arguments. This difference in behaviour is error prone and
doesn't help standardizing per-CPU kthread names.

Unify the behaviours and convert kthread_create_worker_on_cpu() to
use the printf behaviour of kthread_create_on_cpu().

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>