Fix bug identified by Srivatsa Vaddagiri <vatsa@in.ibm.com>:

There's a deadlock in __create_workqueue when CONFIG_HOTPLUG_CPU is set.  This
can happen when create_workqueue_thread fails to create a worker thread.  In
that case, we call destroy_workqueue with cpu hotplug lock held.
destroy_workqueue however also attempts to take the same lock.