filter: Cease out "struct bsc_nat" from the API

This means we need to require a talloc context and
simply operate on the list. I had considered creating
a structure to hold the list head but I didn't find
any other members so omitted it for now.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c b/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
index 89b9742..3a53dd4 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
@@ -1171,7 +1171,7 @@
 	install_element(NAT_NODE, &cfg_nat_ussd_local_cmd);
 	install_element(NAT_NODE, &cfg_nat_use_ipa_for_mgcp_cmd);
 
-	bsc_nat_lst_vty_init(nat, NAT_NODE);
+	bsc_nat_lst_vty_init(nat, &nat->access_lists, NAT_NODE);
 
 	/* number rewriting */
 	install_element(NAT_NODE, &cfg_nat_number_rewrite_cmd);