Do not round per_outbound_htlc_counterparty_commit_tx_fee_msat

The goal is to align `per_outbound_htlc_counterparty_commit_tx_fee_msat`
with the slope `commit_tx_fee_sat` / `num_htlcs`.

This is a minor detail - no existing LDK code multiplies the value
returned by `per_outbound_htlc_counterparty_commit_tx_fee_msat` with
`num_htlcs`.

Author: tankyleo

lightning/src/ln/chan_utils.rs

@@ -197,9 +197,8 @@ pub(crate) fn commit_tx_fee_sat(feerate_per_kw: u32, num_htlcs: usize, channel_t
 }
 
 pub(crate) fn per_outbound_htlc_counterparty_commit_tx_fee_msat(feerate_per_kw: u32, channel_type_features: &ChannelTypeFeatures) -> u64 {
-	// Note that we need to divide before multiplying to round properly,
-	// since the lowest denomination of bitcoin on-chain is the satoshi.
-	let commitment_tx_fee_msat = COMMITMENT_TX_WEIGHT_PER_HTLC * feerate_per_kw as u64 / 1000 * 1000;
+	// We avoid rounding here to make the returned value consistent with the slope (commit_tx_fee_sat/num_htlcs)
+	let commitment_tx_fee_msat = COMMITMENT_TX_WEIGHT_PER_HTLC * feerate_per_kw as u64;
 	if channel_type_features.supports_anchors_zero_fee_htlc_tx() {
 		commitment_tx_fee_msat
 	} else {