vt: avoid a VLA in the unicode screen scroll function

The nr argument is typically small: most often nr == 1. However this
could be abused with a very large explicit scroll in a resized screen.
Make the code scroll lines by performing an array rotation operation to
avoid the need for a large temporary space.

Requested-by: Kees Cook <keescook@chromium.org>
Suggested-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>