Fix route cache flush event
 

This would fix at least the generally observable half of the problem
when doing some profisioning in LXC containers.
The other half looks (after most recent comments) like it could be
specific to certain hardware (or at least a certain net setup).

In upstream this silently was done by the followign two patches:
- net: remove delay at device dismantle
- net: reinstate rtnl in call_netdevice_notifiers()

Both end up on removing the check on in_dev completely. And maybe
the agreed stable solution will be to remove the check altogether.
But for now I wanted to keep the remaining cases unmodified and just
moved the NETDEV_UNREGISTER_BATCH handling in from of the check (like
the NETDEV_UNREGISTER case already is).

One thing is odd, though: The event handler was exactly the same back
in Precise. Still my impression was that nobody had the same issue
back there... still it does sound like the inet dev pointer would not
be set anymore on an event that is done as one of the last steps of
unregistering.

-Stefan