fbdev: fbmem: Fix the implicit type casting

In function do_fb_ioctl(), the "arg" is the type of unsigned long,
and in "case FBIOBLANK:" this argument is casted into an int before
passig to fb_blank(). In fb_blank(), the comparision
if (blank > FB_BLANK_POWERDOWN) would be bypass if the original
"arg" is a large number, which is possible because it comes from
the user input. Fix this by adding the check before the function
call.

Reviewed-by: Guenter Roeck <[e-mail]>
Reviewed-by: Sam Ravnborg <[e-mail]>
Acked-by: Helge Deller <[e-mail]>
Signed-off-by: Yizhuo Zhai <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/video/fbdev/core/fbmem.c

···
 		ret = fbcon_set_con2fb_map_ioctl(argp);
 		break;
 	case FBIOBLANK:
+		if (arg > FB_BLANK_POWERDOWN)
+			return -EINVAL;
 		console_lock();
 		lock_fb_info(info);
 		ret = fb_blank(info, arg);