drm: fix NULL pointer access by wrong ioctl

If user uses wrong ioctl command with _IOC_NONE and argument size
greater than 0, it can cause NULL pointer access from memset of line
463. If _IOC_NONE, don't memset to 0 for kdata.

Signed-off-by: Zhaowei Yuan <zhaowei.yuan@samsung.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_drv.c

@@ -419,8 +419,9 @@ long drm_ioctl(struct file *filp,
 			retcode = -EFAULT;
 			goto err_i1;
 		}
-	} else
+	} else if (cmd & IOC_OUT) {
 		memset(kdata, 0, usize);
+	}
 
 	if (ioctl->flags & DRM_UNLOCKED)
 		retcode = func(dev, kdata, file_priv);