nat: Store the IMSI inside the SCCP Connection data

Store the IMSI for the connections that we are tracking,
it will be freed when the normnal SCCP connection is freed.

openbsc/include/openbsc/bsc_nat_sccp.h

@@ -79,6 +79,7 @@ struct sccp_connections {
 	int con_type;
 	int con_local;
 	int imsi_checked;
+	char *imsi;
 
 	/*
 	 * audio handling. Remember if we have ever send a CRCX,

openbsc/src/nat/bsc_nat_utils.c

@@ -469,6 +469,7 @@ static int _dt_check_id_resp(struct bsc_connection *bsc,
 
 	ret = auth_imsi(bsc, mi_string);
 	con->imsi_checked = 1;
+	con->imsi = talloc_strdup(con, mi_string);
 	return ret;
 }