make freeing super_block rcu-delayed

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b09e4e1..2ab8a67 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1330,6 +1330,7 @@
 	 */
 	struct list_lru		s_dentry_lru ____cacheline_aligned_in_smp;
 	struct list_lru		s_inode_lru ____cacheline_aligned_in_smp;
+	struct rcu_head		rcu;
 };
 
 extern struct timespec current_fs_time(struct super_block *sb);