encoding/json/v2: WithinArshalCall check does not work for nested calls

[dsnet]

There's an internal WithinArshalCall flag that the "json" implementation sets whenever calling a user-provided marshaler or unmarshaler. The purpose of this flag is as a sanity check that causes Encoder.Reset and Decoder.Reset to panic.

This check doesn't work for nested marshaler calls since inner call will clear the flag. We should either drop the check (since it's only as a sanity protection) or fix it such that it only clears the flag if it was able to set it in the first place.

Thus instead of:

https://github.com/go-json-experiment/json/blob/60a0516c28953eb838073b7951a726fa12003537/arshal_methods.go#L195-L197

It'd be something like:

if !xe.Flags.Get(jsonflags.WithinArshalCall) {
     xe.Flags.Set(jsonflags.WithinArshalCall | 1) 
     defer func() { xe.Flags.Set(jsonflags.WithinArshalCall | 0) }
}
err := va.Addr().Interface().(MarshalerTo).MarshalJSONTo(enc, mo) 

Overall, this is a pretty minor bug.