Revert a small part of the earlier adjout_prefix_dump diff.

In up_process_prefix() bring back the path_id_tx hack for regular peers.
A peer not using add-path send will set the path_id_tx to 0 and so
adjout_prefix_first() will return the right match.

This was undone because there is a problem with this when a peer switches
mode (from add-path send back to no add-path). adjout_prefix_first()
returns a prefix where path_id_tx is not 0 but adjout_prefix_update()
expects that to be 0. This edge case is far less common and a better
workaround can be found here.

Fixes the addpath intergration test.

usr.sbin/bgpd/rde_update.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: rde_update.c,v 1.197 2026/05/28 09:10:22 claudio Exp $ */
+/*	$OpenBSD: rde_update.c,v 1.198 2026/05/28 14:01:46 claudio Exp $ */
 
 /*
  * Copyright (c) 2004 Claudio Jeker <claudio@openbsd.org>
@@ -164,6 +164,7 @@ up_process_prefix(struct rde_peer *peer, struct prefix *new,
 	struct filterstate state;
 	struct bgpd_addr addr;
 	int excluded = 0;
+	uint32_t new_path_id_tx = 0;
 
 	/*
 	 * up_test_update() needs to run before the output filters
@@ -194,11 +195,13 @@ up_process_prefix(struct rde_peer *peer, struct prefix *new,
 	}
 
 	/* from here on we know this is an update */
-	if (p == (void *)-1)
+	if (p == (void *)-1) {
+		new_path_id_tx = new->path_id_tx;
 		p = adjout_prefix_get(peer, new->path_id_tx, new->pt);
+	}
 
 	up_prep_adjout(peer, &state, new->pt->aid);
-	adjout_prefix_update(p, peer, &state, new->pt, new->path_id_tx,
+	adjout_prefix_update(p, peer, &state, new->pt, new_path_id_tx,
 	    force_update);
 	rde_filterstate_clean(&state);