Fix cache leak in umount simplefs

1. When the cache be destoryed, we should call rcu_barrier() to prevent
call_rcu() still works and this also prevent the cache be reused.

2. After simplefs_destroy_inode_cache() function, we need rcu_barrier()
to make sure all memory have be freed.

fs.c

@@ -58,6 +58,8 @@ static int __init simplefs_init(void)
 
 err_inode:
     simplefs_destroy_inode_cache();
+    /* Only after rcu_barrier() is the memory guaranteed to be freed. */
+    rcu_barrier();
 err:
     return ret;
 }
@@ -69,6 +71,8 @@ static void __exit simplefs_exit(void)
         pr_err("Failed to unregister file system\n");
 
     simplefs_destroy_inode_cache();
+    /* Only after rcu_barrier() is the memory guaranteed to be freed. */
+    rcu_barrier();
 
     pr_info("module unloaded\n");
 }

super.c

@@ -37,6 +37,9 @@ int simplefs_init_inode_cache(void)
 /* De-allocate the inode cache */
 void simplefs_destroy_inode_cache(void)
 {
+    /* wait for call_rcu() and prevent the free cache be used */
+    rcu_barrier();
+
     kmem_cache_destroy(simplefs_inode_cache);
 }