routing: use updated sphinx.NewOnionPacket method

In this commit, we update the routing package to use the new
`sphinx.NewOnionPacket` method. The new version of this method allows us
to specify _how_ the packet should be filled before it's used to create
a mix-header. This isn't a fundamental change (totally backwards
compatible), instead it plugs a privacy leak that may have revealed to
the destination how long the true route was.

routing/router.go

@@ -1518,6 +1518,7 @@ func generateSphinxPacket(rt *route.Route, paymentHash []byte,
 	// privacy preserving source routing across the network.
 	sphinxPacket, err := sphinx.NewOnionPacket(
 		sphinxPath, sessionKey, paymentHash,
+		sphinx.DeterministicPacketFiller,
 	)
 	if err != nil {
 		return nil, nil, err