Add donut benchmark for 64MB, 128MB, 256MB, 512MB

Current average values

~~~
> minio/pkg/storage/donut/v1 *> go test -bench .
OK: 2 passed
PASS
BenchmarkDonut64M              5         266249634 ns/op         252.05 MB/s
BenchmarkDonut128M             3         436507394 ns/op         307.48 MB/s
BenchmarkDonut256M             2         836116359 ns/op         321.05 MB/s
BenchmarkDonut512M             1        1676738951 ns/op         320.19 MB/s
ok      github.com/minio-io/minio/pkg/storage/donut/v1  8.430s
> minio/pkg/storage/donut/v1 *>
~~~

pkg/storage/donut/v1/donut_test.go

@@ -130,3 +130,29 @@ func (s *MySuite) TestLengthMismatchInWrite(c *C) {
 	err := Write(&testData, bytes.NewBufferString("hello, world"), 5)
 	c.Assert(err, Not(IsNil))
 }
+
+var buf = make([]byte, 1024*1024*1024)
+
+func benchmarkSize(b *testing.B, size int) {
+	b.SetBytes(int64(size))
+	target := new(bytes.Buffer)
+	for i := 0; i < b.N; i++ {
+		Write(target, bytes.NewReader(buf[:size]), uint64(size))
+	}
+}
+
+func BenchmarkDonut64M(b *testing.B) {
+	benchmarkSize(b, 1024*1024*64)
+}
+
+func BenchmarkDonut128M(b *testing.B) {
+	benchmarkSize(b, 1024*1024*128)
+}
+
+func BenchmarkDonut256M(b *testing.B) {
+	benchmarkSize(b, 1024*1024*256)
+}
+
+func BenchmarkDonut512M(b *testing.B) {
+	benchmarkSize(b, 1024*1024*512)
+}