net: introduce pre-change upper device notifier

net: introduce pre-change upper device notifier

This newly introduced netdevice notifier is called before actual change
upper happens. That provides a possibility for notifier handlers to
know upper change will happen and react to it, including possibility to
forbid the change. That is valuable for drivers which can check if the
upper device linkage is supported and forbid that in case it is not.

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