offers: add back extra cltv expiry delta

We had removed this expiry delta because payments to more recent versions of
LDK and CLN were working without it. But without it, some payments to other
versions fail.

src/lndk_offers.rs

@@ -603,7 +603,7 @@ impl InvoicePayer for Client {
 
         let blinded_payment_paths = tonic_lnd::lnrpc::BlindedPaymentPath {
             blinded_path,
-            total_cltv_delta: u32::from(cltv_expiry_delta),
+            total_cltv_delta: u32::from(cltv_expiry_delta) + 120,
             base_fee_msat: u64::from(fee_base_msat),
             proportional_fee_msat: u64::from(fee_ppm),
             ..Default::default()