Correctly apply penalty bounds on the per-amount penalties

When we attempt to score a channel which has a success probability
very low, we may have a log well above our cut-off of two. For the
liquidity penalties this works great, we bound it by
`NEGATIVE_LOG10_UPPER_BOUND` and `min` the two scores. For the
amount liquidity penalty we didn't do any `min`ing at all.

This fix is to min the log itself first and then reuse the min'd
log in both calculations.

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -1088,14 +1088,16 @@ impl<L: Deref<Target = u64>, BRT: Deref<Target = HistoricalBucketRangeTracker>,
 
 	/// Computes the liquidity penalty from the penalty multipliers.
 	#[inline(always)]
-	fn combined_penalty_msat(amount_msat: u64, negative_log10_times_2048: u64,
+	fn combined_penalty_msat(amount_msat: u64, mut negative_log10_times_2048: u64,
 		liquidity_penalty_multiplier_msat: u64, liquidity_penalty_amount_multiplier_msat: u64,
 	) -> u64 {
+		negative_log10_times_2048 =
+			negative_log10_times_2048.min(NEGATIVE_LOG10_UPPER_BOUND * 2048);
+
 		let liquidity_penalty_msat = {
 			// Upper bound the liquidity penalty to ensure some channel is selected.
 			let multiplier_msat = liquidity_penalty_multiplier_msat;
-			let max_penalty_msat = multiplier_msat.saturating_mul(NEGATIVE_LOG10_UPPER_BOUND);
-			(negative_log10_times_2048.saturating_mul(multiplier_msat) / 2048).min(max_penalty_msat)
+			negative_log10_times_2048.saturating_mul(multiplier_msat) / 2048
 		};
 		let amount_penalty_msat = negative_log10_times_2048
 			.saturating_mul(liquidity_penalty_amount_multiplier_msat)