video: uvesafb: Fix integer overflow in allocation

commit 9f645bcc566a1e9f921bdae7528a01ced5bc3713 upstream.

cmap->len can get close to INT_MAX/2, allowing for an integer overflow in
allocation. This uses kmalloc_array() instead to catch the condition.

Reported-by: Dr Silvio Cesare of InfoSect <silvio.cesare@gmail.com>
Fixes: 8bdb3a2d ("uvesafb: the driver core")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>