nat: Factor out the config by token search

In the upcoming authentication improvements it is nice to
separate the finding of the config from the post-allow
handling of it.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat.c b/openbsc/src/osmo-bsc_nat/bsc_nat.c
index 537001e..2f186b2 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat.c
@@ -980,27 +980,21 @@
 		return;
 	}
 
-	llist_for_each_entry(conf, &bsc->nat->bsc_configs, entry) {
-		/*
-		 * Add the '\0' of the token for the memcmp, the IPA messages
-		 * for some reason added null termination.
-		 */
-		const int token_len = strlen(conf->token) + 1;
-
-		if (token_len == len && memcmp(conf->token, token, token_len) == 0) {
-			rate_ctr_inc(&conf->stats.ctrg->ctr[BCFG_CTR_NET_RECONN]);
-			bsc->authenticated = 1;
-			bsc->cfg = conf;
-			osmo_timer_del(&bsc->id_timeout);
-			LOGP(DNAT, LOGL_NOTICE, "Authenticated bsc nr: %d on fd %d\n",
-			     conf->nr, bsc->write_queue.bfd.fd);
-			start_ping_pong(bsc);
-			return;
-		}
+	conf = bsc_config_by_token(bsc->nat, token, len);
+	if (!conf) {
+		LOGP(DNAT, LOGL_ERROR,
+			"No bsc found for token '%s' on fd: %d.\n", token,
+			bsc->write_queue.bfd.fd);
+		return;
 	}
 
-	LOGP(DNAT, LOGL_ERROR, "No bsc found for token '%s' on fd: %d.\n", token,
-	     bsc->write_queue.bfd.fd);
+	rate_ctr_inc(&conf->stats.ctrg->ctr[BCFG_CTR_NET_RECONN]);
+	bsc->authenticated = 1;
+	bsc->cfg = conf;
+	osmo_timer_del(&bsc->id_timeout);
+	LOGP(DNAT, LOGL_NOTICE, "Authenticated bsc nr: %d on fd %d\n",
+		conf->nr, bsc->write_queue.bfd.fd);
+	start_ping_pong(bsc);
 }
 
 static void handle_con_stats(struct nat_sccp_connection *con)