devlink: change port event netdev notifier from per-net to global

Currently only the network namespace of devlink instance is monitored
for port events. If netdev is moved to a different namespace and then
unregistered, NETDEV_PRE_UNINIT is missed which leads to trigger
following WARN_ON in devl_port_unregister().
WARN_ON(devlink_port->type != DEVLINK_PORT_TYPE_NOTSET);

Fix this by changing the netdev notifier from per-net to global so no
event is missed.

Fixes: 02a68a47

 ("net: devlink: track netdev with devlink_port assigned")
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230206094151.2557264-1-jiri@resnulli.us
Signed-off-by: Paolo Abeni <pabeni@redhat.com>