make freeing super_block rcu-delayed

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/super.c b/fs/super.c
index efa6e48..743bb71 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -149,7 +149,7 @@
 	WARN_ON(!list_empty(&s->s_mounts));
 	kfree(s->s_subtype);
 	kfree(s->s_options);
-	kfree(s);
+	kfree_rcu(s, rcu);
 }
 
 /**