drm/drm_crtc: return -EFAULT on copy_to_user errors

copy_from_user() returns the number of bytes left to be copied but we
want to return a negative error code here.  This is in the ioctl handler
so the error code get returned to userspace.
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc.c

@@ -1840,8 +1840,10 @@ int drm_mode_dirtyfb_ioctl(struct drm_device *dev,
 
 		ret = copy_from_user(clips, clips_ptr,
 				     num_clips * sizeof(*clips));
-		if (ret)
+		if (ret) {
+			ret = -EFAULT;
 			goto out_err2;
+		}
 	}
 
 	if (fb->funcs->dirty) {