replace io.Discard usage to fix some NUMA copy() latencies (#18394)

replace io.Discard usage to fix NUMA copy() latencies

On NUMA systems copying from 8K buffer allocated via
io.Discard leads to large latency build-up for every

```
copy(new8kbuf, largebuf)
```

can in-cur upto 1ms worth of latencies on NUMA systems
due to memory sharding across NUMA nodes.

cmd/perf-tests.go

@@ -34,6 +34,7 @@ import (
 	"github.com/minio/madmin-go/v3"
 	"github.com/minio/minio-go/v7"
 	xhttp "github.com/minio/minio/internal/http"
+	xioutil "github.com/minio/minio/internal/ioutil"
 	"github.com/minio/pkg/v2/randreader"
 )
 
@@ -148,6 +149,8 @@ func selfSpeedTest(ctx context.Context, opts speedTestOpts) (SpeedTestResult, er
 	var downloadTimes madmin.TimeDurations
 	var downloadTTFB madmin.TimeDurations
 	wg.Add(opts.concurrency)
+
+	c := minio.Core{Client: globalMinioClient}
 	for i := 0; i < opts.concurrency; i++ {
 		go func(i int) {
 			defer wg.Done()
@@ -161,7 +164,8 @@ func selfSpeedTest(ctx context.Context, opts speedTestOpts) (SpeedTestResult, er
 				}
 				tmpObjName := pathJoin(objNamePrefix, fmt.Sprintf("%d/%d", i, j))
 				t := time.Now()
-				r, err := globalMinioClient.GetObject(downloadsCtx, opts.bucketName, tmpObjName, gopts)
+
+				r, _, _, err := c.GetObject(downloadsCtx, opts.bucketName, tmpObjName, gopts)
 				if err != nil {
 					errResp, ok := err.(minio.ErrorResponse)
 					if ok && errResp.StatusCode == http.StatusNotFound {
@@ -178,7 +182,7 @@ func selfSpeedTest(ctx context.Context, opts speedTestOpts) (SpeedTestResult, er
 				fbr := firstByteRecorder{
 					r: r,
 				}
-				n, err := io.Copy(io.Discard, &fbr)
+				n, err := xioutil.Copy(xioutil.Discard, &fbr)
 				r.Close()
 				if err == nil {
 					response := time.Since(t)