use buffers for pathJoin, to re-use buffers. (#17960)

```
benchmark                        old ns/op     new ns/op     delta
BenchmarkPathJoin/PathJoin-8     79.6          55.3          -30.53%

benchmark                        old allocs     new allocs     delta
BenchmarkPathJoin/PathJoin-8     2              1              -50.00%

benchmark                        old bytes     new bytes     delta
BenchmarkPathJoin/PathJoin-8     48            24            -50.00%
```

cmd/object-api-utils_test.go

@@ -24,6 +24,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"path"
 	"reflect"
 	"runtime"
 	"strconv"
@@ -36,6 +37,38 @@ import (
 	"github.com/minio/pkg/trie"
 )
 
+func pathJoinOld(elem ...string) string {
+	trailingSlash := ""
+	if len(elem) > 0 {
+		if hasSuffixByte(elem[len(elem)-1], SlashSeparatorChar) {
+			trailingSlash = SlashSeparator
+		}
+	}
+	return path.Join(elem...) + trailingSlash
+}
+
+func BenchmarkPathJoinOld(b *testing.B) {
+	b.Run("PathJoin", func(b *testing.B) {
+		b.ResetTimer()
+		b.ReportAllocs()
+
+		for i := 0; i < b.N; i++ {
+			pathJoinOld("volume", "path/path/path")
+		}
+	})
+}
+
+func BenchmarkPathJoin(b *testing.B) {
+	b.Run("PathJoin", func(b *testing.B) {
+		b.ResetTimer()
+		b.ReportAllocs()
+
+		for i := 0; i < b.N; i++ {
+			pathJoin("volume", "path/path/path")
+		}
+	})
+}
+
 // Wrapper
 func TestPathTraversalExploit(t *testing.T) {
 	if runtime.GOOS != globalWindowsOSName {