SQUASH: fbmem: Use fallthrough keyword

Avoid a compiler warning by using the "fallthrough" pseudo-keyword in
place of the old "/* fall through */" comment convention.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

drivers/video/fbdev/core/fbmem.c

@@ -1201,7 +1201,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
 			ret = fb_copyarea_user(info, &copy);
 			break;
 		}
-		/* fall through */
+		fallthrough;
 	default:
 		lock_fb_info(info);
 		fb = info->fbops;