We weren't binding new worker threads to their cpu when onlining.  Using
preempt and the debug version of smp_processor_id found this.
Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>