net: introduce per-netns netdevice notifiers

Often the code for example in drivers is interested in getting notifier
call only from certain network namespace. In addition to the existing
global netdevice notifier chain introduce per-netns chains and allow
users to register to that. Eventually this would eliminate unnecessary
overhead in case there are many netdevices in many network namespaces.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>