XTLS: More separate uplink/downlink flags for splice copy (#4407)

- In 03131c72db new flags were added for uplink/downlink, but that was not suffcient
- Now that the traffic state contains all possible info
- Each inbound and outbound is responsible to set their own CanSpliceCopy flag. Note that this also open up more splice usage. E.g. socks in -> freedom out
- Fixes https://github.com/XTLS/Xray-core/issues/4033

proxy/socks/server.go

@@ -199,6 +199,7 @@ func (s *Server) transport(ctx context.Context, reader io.Reader, writer io.Writ
 	}
 
 	responseDone := func() error {
+		inbound.CanSpliceCopy = 1
 		defer timer.SetTimeout(plcy.Timeouts.UplinkOnly)
 
 		v2writer := buf.NewWriter(writer)
@@ -256,6 +257,7 @@ func (s *Server) handleUDPPayload(ctx context.Context, conn stat.Connection, dis
 	if inbound != nil && inbound.Source.IsValid() {
 		errors.LogInfo(ctx, "client UDP connection from ", inbound.Source)
 	}
+	inbound.CanSpliceCopy = 1
 
 	var dest *net.Destination