cache: assert filter when calling nft_cache_evaluate()

cache: assert filter when calling nft_cache_evaluate()

nft_cache_evaluate() always takes a non-null filter, remove superfluous
checks when calculating cache requirements via flags.

Note that filter is still option from netlink dump path, since this can
be called from error path to provide hints.

Fixes: 08725a9dc14c ("cache: filter out rules by chain")
Fixes: b3ed8fd8c9f3 ("cache: missing family in cache filtering")
Fixes: 635ee1cad8aa ("cache: filter out sets and maps that are not requested")
Fixes: 3f1d3912c3a6 ("cache: filter out tables that are not requested")
Tested-by: Eric Garver <eric@garver.life>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>