Add unregister_fib_notifier as rollback of register_fib_notifier.

Fixes: 4394fbcb ("net: marvell: prestera: handle fib notifications")
Signed-off-by: NYevhen Orlov <yevhen.orlov@plvision.eu>
Link: https://lore.kernel.org/r/20220710122021.7642-1-yevhen.orlov@plvision.euSigned-off-by: NPaolo Abeni <pabeni@redhat.com>