Fix `Route` serialization round-trip

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.