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

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>
Cc: stable <stable@kernel.org> [3.3+]
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_page_alloc_dma.c

@@ -1015,6 +1015,8 @@ ttm_dma_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 		return SHRINK_STOP;
 
 	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;
@@ -1034,6 +1036,7 @@ ttm_dma_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 			 p->pool->dev_name, p->pool->name, current->pid,
 			 nr_free, shrink_pages);
 	}
+out:
 	mutex_unlock(&_manager->lock);
 	return freed;
 }