Fix Route serialization round-trip

When the `max_total_routing_fee_msat` parameter was added to
`RouteParameters`, the serialization used `map` to get the max fee,
accidentally writing an `Option<Option<u64>>`, but then read it as
an `Option<u64>`. Thus, any `Route`s with a `route_params` written
will fail to be read back.

Luckily, this is an incredibly rarely-used bit of code, so only one
user managed to hit it.

Author: TheBlueMatt

lightning/src/routing/router.rs

@@ -529,7 +529,7 @@ impl Writeable for Route {
 			(1, self.route_params.as_ref().map(|p| &p.payment_params), option),
 			(2, blinded_tails, optional_vec),
 			(3, self.route_params.as_ref().map(|p| p.final_value_msat), option),
-			(5, self.route_params.as_ref().map(|p| p.max_total_routing_fee_msat), option),
+			(5, self.route_params.as_ref().and_then(|p| p.max_total_routing_fee_msat), option),
 		});
 		Ok(())
 	}