cgroups: Fix a lockdep warning at cgroup removal

Commit 2fd6b7f5 ("fs: dcache scale subdirs") forgot to annotate a dentry
lock, which caused a lockdep warning.
Reported-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

kernel/cgroup.c

@@ -912,7 +912,7 @@ static void cgroup_d_remove_dir(struct dentry *dentry)
 
 	parent = dentry->d_parent;
 	spin_lock(&parent->d_lock);
-	spin_lock(&dentry->d_lock);
+	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 	list_del_init(&dentry->d_u.d_child);
 	spin_unlock(&dentry->d_lock);
 	spin_unlock(&parent->d_lock);