nft: ensure to assign rule id only once

Make sure an assigned id is not overwritten.

So far this function was guaranteed to be called only once. But I would
like to allow getting the nftables ruleset string more than once in a
future patch. Prepare that.

Change-Id: I4e8c48c01fb2f5d4cfd223fe03abbf15b1a55670

include/osmocom/upf/upf_nft.h

@@ -42,6 +42,7 @@ struct upf_nft_tunmap_desc {
 		struct osmo_sockaddr gtp_remote_addr;
 		uint32_t remote_teid;
 	} core;
+	/* id as in ruleset name 'tunmap<id>'. If zero, no id has been assigned yet. */
 	uint32_t id;
 };