netfilter: nfacct: per network namespace support

- Move the nfnl_acct_list into the network namespace, initialize
  and destroy it per namespace
- Keep track of refcnt on nfacct objects, the old logic does not
  longer work with a per namespace list
- Adjust xt_nfacct to pass the namespace when registring objects

Signed-off-by: Andreas Schultz <aschultz@tpip.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>