net: introduce NETDEV_UNREGISTER_PERNET

This new event is called once for each unique net namespace in batched
unregister operations (with the argument set to a random device from
that namespace) and once per device in non-batched unregister
operations.

It allows us to factorize some device unregister work such as clearing the
routing cache.

Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>