From: Rusty Russell <rusty@rustcorp.com.au>

call_usermodehelper uses keventd to create a thread, guaranteeing a nice,
clean kernel thread.  Unfortunately, there is a case where
call_usermodehelper is called with &bus->subsys.rwsem held (via
bus_add_driver()), but keventd could be running bus_add_device(), which is
blocked on the same lock.  The result is deadlock, and it comes from using
keventd for both.

In this case, it can be fixed by using a completely independent thread for
call_usermodehelper, or an independent workqueue.  Workqueues have the
infrastructure we need, so we use one.

Move EXPORT_SYMBOL while we're there, too.

akpm fixes: Make it compile with !CONFIG_KMOD