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>

include/net/net_namespace.h

@@ -118,6 +118,9 @@ struct net {
 #endif
 	struct sock		*nfnl;
 	struct sock		*nfnl_stash;
+#if IS_ENABLED(CONFIG_NETFILTER_NETLINK_ACCT)
+	struct list_head        nfnl_acct_list;
+#endif
 #endif
 #ifdef CONFIG_WEXT_CORE
 	struct sk_buff_head	wext_nlevents;