net: devlink: add WARN_ON_ONCE to check return value of unregister_netdevice_notifier_net() call

As the return value is not 0 only in case there is no such notifier
block registered, add a WARN_ON_ONCE() to yell about it.

Suggested-by: Ido Schimmel <idosch@idosch.org>
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20221125100255.1786741-1-jiri@resnulli.us
Signed-off-by: Paolo Abeni <pabeni@redhat.com>