handlers/objects: Optimize buffer allocation for multipart form file

Continues f1f7ab4 for
`UploadContainerObject`. Size of the file from the multipart form is at
hand, so using it to allocate a buffer will improve performance on small
volumes.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

handlers/objects.go

@@ -1036,8 +1036,15 @@ func (a *RestAPI) UploadContainerObject(ctx echo.Context, containerID apiserver.
 		return ctx.JSON(http.StatusBadRequest, resp)
 	}
 
-	chunk := make([]byte, a.payloadBufferSize)
-	_, err = io.CopyBuffer(writer, file, chunk)
+	var buf []byte
+	if header.Size > 0 && uint64(header.Size) < a.payloadBufferSize {
+		buf = make([]byte, header.Size)
+	} else {
+		// Size field is not documented, so we cannot be sure what exactly non-positive
+		// values mean. Thus, it's better to keep default behavior for them.
+		buf = make([]byte, a.payloadBufferSize)
+	}
+	_, err = io.CopyBuffer(writer, file, buf)
 	if err != nil {
 		resp := a.logAndGetErrorResponse("write", err)
 		return ctx.JSON(http.StatusBadRequest, resp)