schedule_on_each_cpu(): use preempt_disable()

We take workqueue_mutex in there to keep CPU hotplug away.  But
preempt_disable() will suffice for that.

Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>