Allow for BOLT11 overpayments again

Previously, we refactored our BOLT11 payment API which made
invoice-provided and user-provided amounts mutually exclusive.

Here, we restore the original behavior that allows overpaying for
invoices, even if they specify a (default) amount.

Author: tnull

lightning/src/ln/channelmanager.rs

@@ -4864,10 +4864,11 @@ where
 	///
 	/// # Handling Invoice Amounts
 	/// Some invoices include a specific amount, while others require you to specify one.
-	/// - If the invoice **includes** an amount, user must not provide `amount_msats`.
+	/// - If the invoice **includes** an amount, user may provide an amount greater or equal to it
+	/// to allow for overpayments.
 	/// - If the invoice **doesn't include** an amount, you'll need to specify `amount_msats`.
 	///
-	/// If these conditions aren’t met, the function will return `Bolt11PaymentError::InvalidAmount`.
+	/// If these conditions aren’t met, the function will return [`Bolt11PaymentError::InvalidAmount`].
 	///
 	/// # Custom Routing Parameters
 	/// Users can customize routing parameters via [`RouteParametersConfig`].

lightning/src/ln/outbound_payment.rs

@@ -588,8 +588,7 @@ pub(crate) enum PaymentSendFailure {
 #[derive(Debug)]
 pub enum Bolt11PaymentError {
 	/// Incorrect amount was provided to [`ChannelManager::pay_for_bolt11_invoice`].
-	/// This happens when an amount is specified when [`Bolt11Invoice`] already contains
-	/// an amount, or vice versa.
+	/// This happens when the user-provided amount is less than an amount specified in the [`Bolt11Invoice`].
 	///
 	/// [`Bolt11Invoice`]: lightning_invoice::Bolt11Invoice
 	/// [`ChannelManager::pay_for_bolt11_invoice`]: crate::ln::channelmanager::ChannelManager::pay_for_bolt11_invoice
@@ -894,7 +893,9 @@ impl OutboundPayments {
 
 		let amount = match (invoice.amount_milli_satoshis(), amount_msats) {
 			(Some(amt), None) | (None, Some(amt)) => amt,
-			(None, None) | (Some(_), Some(_)) => return Err(Bolt11PaymentError::InvalidAmount),
+			(Some(inv_amt), Some(user_amt)) if user_amt < inv_amt => return Err(Bolt11PaymentError::InvalidAmount),
+			(Some(_), Some(user_amt)) => user_amt,
+			(None, None) => return Err(Bolt11PaymentError::InvalidAmount),
 		};
 
 		let mut recipient_onion = RecipientOnionFields::secret_only(*invoice.payment_secret());