net: marvell: prestera: Add strict cleanup of fib arbiter

This will, ensure, that there is no more, preciously allocated fib_cache
entries left after deinit.
Will be used to free allocated resources of nexthop routes, that points
to "not our" port (e.g. eth0).

Signed-off-by: Yevhen Orlov <yevhen.orlov@plvision.eu>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>