video: uvesafb: Fix integer overflow in allocation

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: 8bdb3a2d7df48 ("uvesafb: the driver core")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
static int uvesafb_helper_start(void)
{
	char *envp[] = {
		"HOME=/",
		"PATH=/sbin:/bin",
		NULL,
	};

	char *argv[] = {
		v86d_path,
		NULL,
	};

	return call_usermodehelper(v86d_path, argv, envp, UMH_WAIT_PROC);
}

static int uvesafb_helper_start(void)
{
	char *envp[] = {
		"HOME=/",
		"PATH=/sbin:/bin",
		NULL,
	};

	char *argv[] = {
		v86d_path,
		NULL,
	};

	return call_usermodehelper(v86d_path, argv, envp, UMH_WAIT_PROC);
}
