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

This is a followup to CL 14177. It applies copyBufPool optimization to
transferWriter.doBodyCopy(). The function is 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
 	}