cache: add NFT_CACHE_UPDATE and NFT_CACHE_FLUSHED flags

cache: add NFT_CACHE_UPDATE and NFT_CACHE_FLUSHED flags

NFT_CACHE_FLUSHED tells cache_update() to skip the netlink dump to
populate the cache, since the existing ruleset is going to flushed by
this batch.

NFT_CACHE_UPDATE tells rule_evaluate() to perform incremental updates to
the cache based on the existing batch, this is required by the rule
commands that use the index and the position selectors.

This patch removes cache_flush() which is not required anymore. This
cache removal is coming too late, in the evaluation phase, after the
initial cache_update() invocation.

Be careful with NFT_CACHE_UPDATE, this flag needs to be left in place if
NFT_CACHE_FLUSHED is set on.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>