There are races WRT walking the lists of the buses and classes and the 
device being unregistered while it's the current node we're accessing. If 
the node was deleted, ->next would point to itself, and we'd sit in an
inifinite loop (until that memory was freed from underneath us and we Oops'd).

This changes device_unregister() to only mark the device as not present. put_device()
deletes the device from all the lists it belongs to, while holding device_lock,
then drops it to call device_del().

This should guarantee that everything in the device is valid until the last reference
goes away, regardless of whether it's still physically present or not.