Use CLTV_FAR_FAR_AWAY in PaymentConstraints

When finding a route through a blinded path, a random CLTV offset may be
added to the path in order to preserve privacy. This needs to be
accounted for in the blinded path's PaymentConstraints. Add
CLTV_FAR_FAR_AWAY to the max_cltv_expiry constraint to allow for such
offsets.

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -7876,7 +7876,8 @@ where
 		let secp_ctx = &self.secp_ctx;
 
 		let payee_node_id = self.get_our_node_id();
-		let max_cltv_expiry = self.best_block.read().unwrap().height() + LATENCY_GRACE_PERIOD_BLOCKS;
+		let max_cltv_expiry = self.best_block.read().unwrap().height() + CLTV_FAR_FAR_AWAY
+			+ LATENCY_GRACE_PERIOD_BLOCKS;
 		let payee_tlvs = ReceiveTlvs {
 			payment_secret,
 			payment_constraints: PaymentConstraints {