fix: use the update_add_htlc's cltv_expiry for comparison

As noted in BOLT 4, we should be using the update_add_htlc's cltv_expiry,
not the CLTV expiry set by the sender in the onion for this comparison.
See here: https://github.com/lightning/bolts/blob/4dcc377209509b13cf89a4b91fde7d478f5b46d8/04-onion-routing.md?plain=1#L334

Author: vladimirfomene

lightning/src/ln/channelmanager.rs

@@ -2935,7 +2935,7 @@ where
 		// payment logic has enough time to fail the HTLC backward before our onchain logic triggers a
 		// channel closure (see HTLC_FAIL_BACK_BUFFER rationale).
 		let current_height: u32 = self.best_block.read().unwrap().height();
-		if (outgoing_cltv_value as u64) <= current_height as u64 + HTLC_FAIL_BACK_BUFFER as u64 + 1 {
+		if (cltv_expiry as u64) <= current_height as u64 + HTLC_FAIL_BACK_BUFFER as u64 + 1 {
 			let mut err_data = Vec::with_capacity(12);
 			err_data.extend_from_slice(&amt_msat.to_be_bytes());
 			err_data.extend_from_slice(&current_height.to_be_bytes());