sfc: Forget filter ID when the filter is marked old

It is required to remove setting of filter IDs to invalid from multicast
and unicast addresses caching functions.
Add initialization to invalid when filter table is created.
Add paranoid checks to track consistency.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>