drm/ttm: fix ttm_bo_add_ttm(user) failure path

ttm_tt_destroy kfrees passed object, so we need to nullify
a reference to it.
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: stable@kernel.org
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_bo.c

@@ -353,8 +353,10 @@ static int ttm_bo_add_ttm(struct ttm_buffer_object *bo, bool zero_alloc)
 
 		ret = ttm_tt_set_user(bo->ttm, current,
 				      bo->buffer_start, bo->num_pages);
-		if (unlikely(ret != 0))
+		if (unlikely(ret != 0)) {
 			ttm_tt_destroy(bo->ttm);
+			bo->ttm = NULL;
+		}
 		break;
 	default:
 		printk(KERN_ERR TTM_PFX "Illegal buffer object type\n");