htlcswitch: modify forwarding fee assertion to compare emperical fees

In this commit, we fix a bug that was uncovered by the recent change to
lnwire.MilliSatoshi. Rather than manually compute the diff in fees,
we’ll directly compare the fee that is given against the fee that we
expect.

htlcswitch/link.go

@@ -1729,15 +1729,18 @@ func (l *channelLink) processLockedInHtlcs(
 					fwdInfo.AmountToForward,
 				)
 
-				// If the amount of the incoming HTLC, minus
-				// our expected fee isn't equal to the
-				// forwarding instructions, then either the
-				// values have been tampered with, or the send
-				// used incorrect/dated information to
+				// If the actual fee is less than our expected
+				// fee, then we'll reject this HTLC as it
+				// didn't provide a sufficient amount of fees,
+				// or the values have been tampered with, or
+				// the send used incorrect/dated information to
 				// construct the forwarding information for
 				// this hop. In any case, we'll cancel this
 				// HTLC.
-				if pd.Amount-expectedFee < fwdInfo.AmountToForward {
+				actualFee := pd.Amount - fwdInfo.AmountToForward
+				if pd.Amount < fwdInfo.AmountToForward ||
+					actualFee < expectedFee {
+
 					log.Errorf("Incoming htlc(%x) has "+
 						"insufficient fee: expected "+
 						"%v, got %v", pd.RHash[:],