benchmark: Correct bufferSize to highWaterMark

The bufferSize has been removed. Use highWaterMark instead of.

It make the benchmark test result incorrect:

```
fs/read-stream-throughput.js type=buf size=1024: 699.70
fs/read-stream-throughput.js type=buf size=4096: 701.41
fs/read-stream-throughput.js type=buf size=65535: 713.00
fs/read-stream-throughput.js type=buf size=1048576: 704.68
```

the real result is:

```
fs/read-stream-throughput.js type=buf size=1024: 51.328
fs/read-stream-throughput.js type=buf size=4096: 161.40
fs/read-stream-throughput.js type=buf size=65535: 679.61
fs/read-stream-throughput.js type=buf size=1048576: 858.80
```