Add NextCommitmentStats::get_balances_including_fee

`NextCommitmentStats` provides the commitment transaction fee as a
separate value to assist with applying a multiplier on it in
`can_accept_incoming_htlc`.

Nonetheless in most cases, we want the balances to include the
commitment transaction fee, so here we add a helper that gives us these
balances.

Author: tankyleo

lightning/src/sign/tx_builder.rs

@@ -35,6 +35,7 @@ impl HTLCAmountDirection {
 }
 
 pub(crate) struct NextCommitmentStats {
+	pub is_outbound_from_holder: bool,
 	pub inbound_htlcs_count: usize,
 	pub inbound_htlcs_value_msat: u64,
 	pub holder_balance_before_fee_msat: Option<u64>,
@@ -48,6 +49,26 @@ pub(crate) struct NextCommitmentStats {
 	pub extra_nondust_htlc_on_counterparty_tx_dust_exposure_msat: Option<u64>,
 }
 
+impl NextCommitmentStats {
+	pub(crate) fn get_balances_including_fee_msat(&self) -> (Option<u64>, Option<u64>) {
+		let holder_balance_incl_fee_msat = if self.is_outbound_from_holder {
+			self.holder_balance_before_fee_msat
+				.and_then(|balance| balance.checked_sub(self.commit_tx_fee_sat * 1000))
+		} else {
+			self.holder_balance_before_fee_msat
+		};
+
+		let counterparty_balance_incl_fee_msat = if self.is_outbound_from_holder {
+			self.counterparty_balance_before_fee_msat
+		} else {
+			self.counterparty_balance_before_fee_msat
+				.and_then(|balance| balance.checked_sub(self.commit_tx_fee_sat * 1000))
+		};
+
+		(holder_balance_incl_fee_msat, counterparty_balance_incl_fee_msat)
+	}
+}
+
 #[rustfmt::skip]
 fn excess_fees_on_counterparty_tx_dust_exposure_msat(
 	next_commitment_htlcs: &[HTLCAmountDirection], dust_buffer_feerate: u32,
@@ -243,6 +264,7 @@ impl TxBuilder for SpecTxBuilder {
 			};
 
 		NextCommitmentStats {
+			is_outbound_from_holder,
 			inbound_htlcs_count,
 			inbound_htlcs_value_msat,
 			holder_balance_before_fee_msat,