benchmarks: fix invalid option of length for iperf benchmark

According to iperf help page:
```
-l, --len       #[kmKM]    length of buffer in bytes to read or write
(Defaults: TCP=128K, v4 UDP=1470, v6 UDP=1450).
```

So "--length 64K" option is invalid for iperf, actually it will be
regard as default option "--len 128K".

Signed-off-by: Chen Hui <cedriccchen@tencent.com>

test/benchmarks/tools/iperf.go

@@ -21,7 +21,7 @@ import (
 	"testing"
 )
 
-const length = 64 * 1024
+const length = 128 * 1024
 
 // Iperf is for the client side of `iperf`.
 type Iperf struct {
@@ -35,7 +35,7 @@ func (i *Iperf) MakeCmd(host string, port int) []string {
 		"--format", "K", // Output in KBytes.
 		"--realtime",                       // Measured in realtime.
 		"--num", fmt.Sprintf("%dK", i.Num), // Number of bytes to send in KB.
-		"--length", fmt.Sprintf("%d", length),
+		"--len", fmt.Sprintf("%d", length),
 		"--client", host,
 		"--port", fmt.Sprintf("%d", port),
 	}