writeback: memcg_blkcg_tree_lock can be static

Fixes: 60448d43 ("writeback: add memcg_blkcg_link tree")
Signed-off-by: Nkbuild test robot <lkp@intel.com>
Signed-off-by: NJiufei Xue <jiufei.xue@linux.alibaba.com>

mm/backing-dev.c

@@ -417,7 +417,7 @@ struct memcg_blkcg_link {
 };
 
 static RADIX_TREE(memcg_blkcg_tree, GFP_ATOMIC);
-DEFINE_SPINLOCK(memcg_blkcg_tree_lock);
+static DEFINE_SPINLOCK(memcg_blkcg_tree_lock);
 
 static int memcg_blkcg_link_show(struct seq_file *m, void *v)
 {