[PATCH] fix deadlock in create_workqueue()

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.

kernel/workqueue.c

@@ -334,6 +334,7 @@ struct workqueue_struct *__create_workqueue(const char *name,
 				destroy = 1;
 		}
 	}
+	unlock_cpu_hotplug();
 
 	/*
 	 * Was there any error during startup? If yes then clean up:
@@ -342,7 +343,6 @@ struct workqueue_struct *__create_workqueue(const char *name,
 		destroy_workqueue(wq);
 		wq = NULL;
 	}
-	unlock_cpu_hotplug();
 	return wq;
 }