make freezeable workqueues singlethread

It is a known fact that freezeable multithreaded workqueues doesn't like
CPU_DEAD. We keep them only for the incoming CPU-hotplug rework.

Sadly, we can't just kill create_freezeable_workqueue() right now, make
them singlethread.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>