We had two major issues when unregistering networking devices.
1) Even trying to run hotplug asynchronously could deadlock
   if keventd was currently trying to get the RTNL semaphore
   in order to process linkwatch events.
2) Unregister needs to wait for the last reference to go away
   before the finalization of the unregister can execute.  This
   cannot occur under the RTNL semaphore as this is deadlock
   prone as well.

The solution is to do all of this stuff after dropping the
RTNL semaphore.  rtnl_lock, if it is about to protect a region
of code that could unregister network devices, registers a list
to which unregistered netdevs are attached.  At rtnl_unlock time
this list is processed to wait for refcounts to drop to zero and
then finalize the unregister.