Oleg explains the reason we could hit park+park is that
smpboot_update_cpumask_percpu_thread()'s

  for_each_cpu_and(cpu, &tmp, cpu_online_mask)
	smpboot_park_kthread();

turns into:

  for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)and)
	smpboot_park_kthread();

on UP, ignoring the mask. But since we just completely removed that
function, this is no longer relevant.

So revert commit:

  b1f5b378 ("kthread: Allow kthread_park() on a parked kthread")
Suggested-by: NOleg Nesterov <oleg@redhat.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NIngo Molnar <mingo@kernel.org>