pay: fix bolt12 blinded path cltv logic.

The spec has moved a bit here: the `outgoing_cltv_value` in the final onion
is basically the blockheight now (plus the 1 block delta we give ourselves).

Also, we were doubling ours, since p->min_final_cltv_expiry was already set
to p->blindedpay->cltv_expiry_delta above.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

plugins/libplugin-pay.c

@@ -1869,11 +1869,24 @@ static void payment_compute_onion_payloads(struct payment *p)
 
 	/* If we're headed to a blinded path, connect that now. */
 	if (root->blindedpath) {
+		/* This final_cltv matches our payment heuristic of adding 1 block. */
+
+		/* BOLT #4:
+		 * - For every node inside a blinded route:
+		 *...
+		 *   - If it is the final node:
+		 *...
+		 *       - The value set for `outgoing_cltv_value`:
+		 *         - MUST use the current block height as a baseline value.
+		 *         - if a [random offset](07-routing-gossip.md#recommendations-for-routing) was added to improve privacy:
+		 *           - SHOULD add the offset to the baseline value.
+		 */
+		u32 final_cltv = p->start_block + 1;
 		payment_add_blindedpath(cr->hops, cr->hops + hopcount - 1,
 					root->blindedpath,
 					root->blindedouramount,
 					root->blindedfinalamount,
-					root->blindedfinalcltv);
+					final_cltv);
 		tal_append_fmt(&routetxt, "%s -> blinded path (%zu hops)",
 			       fmt_short_channel_id(tmpctx,
 						    p->route[hopcount-1].scid),