act_connmark: Remember the struct net instead of guessing it.

Stop guessing the struct net instead of remember it.  Guessing is just
silly and will be problematic in the future when I implement routes
between network namespaces.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>