src: Fix cache_flush() in cache_needs_more() logic

src: Fix cache_flush() in cache_needs_more() logic

Commit 34a20645d54fa enabled cache updates depending on command causing
it. As a side-effect, this disabled measures in cache_flush() preventing
a later cache update. Re-establish this by setting cache->cmd in
addition to cache->genid after dropping cache entries.

While being at it, set cache->cmd in cache_release() as well. This
shouldn't be necessary since zeroing cache->genid should suffice for
cache_update(), but better be consistent (and future-proof) here.

Fixes: eeda228c2d17 ("src: update cache if cmd is more specific")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>