Fix cmp::max execution in ChannelContext::get_dust_buffer_feerate

  The current `cmp::max` doesnt align with the function comment, ie its
  comparing 2530 and `feerate_plus_quarter` instead of `feerate_per_kw
  + 2530` and `feerate_plus_quarter` which is fixed in this commit

Author: jbesraa

lightning/src/ln/channel.rs

@@ -2730,7 +2730,7 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider  {
 			feerate_per_kw = cmp::max(feerate_per_kw, feerate);
 		}
 		let feerate_plus_quarter = feerate_per_kw.checked_mul(1250).map(|v| v / 1000);
-		cmp::max(2530, feerate_plus_quarter.unwrap_or(u32::max_value()))
+		cmp::max(feerate_per_kw + 2530, feerate_plus_quarter.unwrap_or(u32::max_value()))
 	}
 
 	/// Get forwarding information for the counterparty.

lightning/src/ln/functional_tests.rs

@@ -9950,7 +9950,10 @@ fn do_test_max_dust_htlc_exposure(dust_outbound_balance: bool, exposure_breach_e
 	let dust_outbound_htlc_on_holder_tx_msat: u64 = (dust_buffer_feerate * htlc_timeout_tx_weight(&channel_type_features) / 1000 + open_channel.common_fields.dust_limit_satoshis - 1) * 1000;
 	let dust_outbound_htlc_on_holder_tx: u64 = max_dust_htlc_exposure_msat / dust_outbound_htlc_on_holder_tx_msat;
 
-	let dust_inbound_htlc_on_holder_tx_msat: u64 = (dust_buffer_feerate * htlc_success_tx_weight(&channel_type_features) / 1000 + open_channel.common_fields.dust_limit_satoshis - 1) * 1000;
+	// Substract 3 sats for multiplier and 2 sats for fixed limit to make sure we are 50% below the dust limit.
+	// This is to make sure we fully use the dust limit. If we don't, we could end up with `dust_ibd_htlc_on_holder_tx` being 1 
+	// while `max_dust_htlc_exposure_msat` is not equal to `dust_outbound_htlc_on_holder_tx_msat`.
+	let dust_inbound_htlc_on_holder_tx_msat: u64 = (dust_buffer_feerate * htlc_success_tx_weight(&channel_type_features) / 1000 + open_channel.common_fields.dust_limit_satoshis - if multiplier_dust_limit { 3 } else { 2 }) * 1000; 
 	let dust_inbound_htlc_on_holder_tx: u64 = max_dust_htlc_exposure_msat / dust_inbound_htlc_on_holder_tx_msat;
 
 	let dust_htlc_on_counterparty_tx: u64 = 4;