net/http: use copyBufPool in transferWriter.doBodyCopy()

This patch is a followup to 6fd82d8.
It applies copyBufPool optimization to transferWriter.doBodyCopy().
The function is very frequently used - every time Request or Response is written.

Without this patch for every Request and Response processed, if there is a body, we need to allocate and GC a 32k buffer.
This is quickly causing GC pressure.

Fixes #57202

src/net/http/transfer.go

@@ -410,7 +410,11 @@ func (t *transferWriter) writeBody(w io.Writer) (err error) {
 //
 // This function is only intended for use in writeBody.
 func (t *transferWriter) doBodyCopy(dst io.Writer, src io.Reader) (n int64, err error) {
-	n, err = io.Copy(dst, src)
+	bufp := copyBufPool.Get().(*[]byte)
+	buf := *bufp
+	defer copyBufPool.Put(bufp)
+
+	n, err = io.CopyBuffer(dst, src, buf)
 	if err != nil && err != io.EOF {
 		t.bodyReadError = err
 	}