streams2: Correct drain/return logic

It was testing the length *before* adding the current chunk, which
is the opposite of correct.

Also, the return value was flipped.

lib/_stream_writable.js

@@ -71,13 +71,13 @@ Writable.prototype.write = function(chunk, encoding) {
   if (typeof chunk === 'string')
     chunk = new Buffer(chunk, encoding);
 
-  var ret = state.length >= state.highWaterMark;
-  if (ret === false)
-    state.needDrain = true;
-
   var l = chunk.length;
   state.length += l;
 
+  var ret = state.length < state.highWaterMark;
+  if (ret === false)
+    state.needDrain = true;
+
   if (state.writing) {
     state.buffer.push(chunk);
     return ret;