htlcswitch: for UpdateFailMalformedHTLC packets mark fail as needing conversion

In this commit, we start the first phase of fixing an existing bug
within the switch. As is, we don't properly convert
`UpdateFailMalformedHTLC` to regular `UpdateFailHTLC` messages that are
fully encrypted. When we receive a `UpdateFailMalformedHTLC` today,
we'll convert it into a regular fail message by simply encoding the
failure message raw. This failure message doesn't have  a MAC yet, so if
we sent it backwards, then the destination wouldn't be able to decrypt
it. We can recognize this type of failure as it'll be the same size as
the raw failure message max size, but it has 4 extra bytes for the
encoding. When we come across this message, we'll mark is as needing
conversion so the switch can take care of it.

htlcswitch/link.go

@@ -2369,10 +2369,24 @@ func (l *channelLink) processRemoteSettleFails(fwdPkg *channeldb.FwdPkg,
 				outgoingHTLCID: pd.ParentIndex,
 				destRef:        pd.DestRef,
 				htlc: &lnwire.UpdateFailHTLC{
-					Reason: lnwire.OpaqueReason(pd.FailReason),
+					Reason: lnwire.OpaqueReason(
+						pd.FailReason,
+					),
 				},
 			}
 
+			// If the failure message lacks an HMAC (but includes
+			// the 4 bytes for encoding the message and padding
+			// lengths, then this means that we received it as an
+			// UpdateFailMalformedHTLC. As a result, we'll signal
+			// that we need to convert this error within the switch
+			// to an actual error, by encrypting it as if we were
+			// the originating hop.
+			convertedErrorSize := lnwire.FailureMessageLength + 4
+			if len(pd.FailReason) == convertedErrorSize {
+				failPacket.convertedError = true
+			}
+
 			// Add the packet to the batch to be forwarded, and
 			// notify the overflow queue that a spare spot has been
 			// freed up within the commitment state.