usbmon registers the notifier chain, takes the bus lock and then goes to
scan the existing devices for hooking up.

Unfortunately, if usb_mon gets initialized while USB bus discovery is
going on, it's possible that usbmon gets a notifier on one cpu (which runs
without USB locks), and the scan is going on and also finds the new bus,
resulting in a double sysfs registration, which then produces a WARNING.

Pete Zaitcev did the bug diagnostics on this one
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Acked-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>