drm/ttm: Fix possible division by 0 in ttm_dma_pool_shrink_scan().

commit 11e504cc upstream.

list_empty(&_manager->pools) being false before taking _manager->lock
does not guarantee that _manager->npools != 0 after taking _manager->lock
because _manager->npools is updated under _manager->lock.
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/ttm/ttm_page_alloc_dma.c

@@ -1016,6 +1016,8 @@ static int ttm_dma_pool_mm_shrink(struct shrinker *shrink,
 		return 0;
 
 	mutex_lock(&_manager->lock);
+	if (!_manager->npools)
+		goto out;
 	pool_offset = pool_offset % _manager->npools;
 	list_for_each_entry(p, &_manager->pools, pools) {
 		unsigned nr_free;
@@ -1033,6 +1035,7 @@ static int ttm_dma_pool_mm_shrink(struct shrinker *shrink,
 			 p->pool->dev_name, p->pool->name, current->pid,
 			 nr_free, shrink_pages);
 	}
+out:
 	mutex_unlock(&_manager->lock);
 	/* return estimated number of unused pages in pool */
 	return ttm_dma_pool_get_num_unused_pages();