f2fs: fix race condition in between free nid allocator/initializer

static bool add_free_nid(struct f2fs_sb_info *sbi, nid_t nid, bool build)

{

	struct f2fs_nm_info *nm_i = NM_I(sbi);

	struct free_nid *i;

	struct free_nid *i, *e;

	struct nat_entry *ne;

	int err;

	int err = -EINVAL;

	bool ret = false;

	/* 0 nid should not be used */

	if (unlikely(nid == 0))

		return false;

	if (build) {

		/* do not add allocated nids */

		ne = __lookup_nat_cache(nm_i, nid);

		if (ne && (!get_nat_flag(ne, IS_CHECKPOINTED) ||

				nat_get_blkaddr(ne) != NULL_ADDR))

			return false;

	}

	i = f2fs_kmem_cache_alloc(free_nid_slab, GFP_NOFS);

	i->nid = nid;

	i->state = NID_NEW;

	if (radix_tree_preload(GFP_NOFS)) {

		kmem_cache_free(free_nid_slab, i);

		return true;

	}

	if (radix_tree_preload(GFP_NOFS))

		goto err;

	spin_lock(&nm_i->nid_list_lock);

	if (build) {

		/*

		 *   Thread A             Thread B

		 *  - f2fs_create

		 *   - f2fs_new_inode

		 *    - alloc_nid

		 *     - __insert_nid_to_list(ALLOC_NID_LIST)

		 *                     - f2fs_balance_fs_bg

		 *                      - build_free_nids

		 *                       - __build_free_nids

		 *                        - scan_nat_page

		 *                         - add_free_nid

		 *                          - __lookup_nat_cache

		 *  - f2fs_add_link

		 *   - init_inode_metadata

		 *    - new_inode_page

		 *     - new_node_page

		 *      - set_node_addr

		 *  - alloc_nid_done

		 *   - __remove_nid_from_list(ALLOC_NID_LIST)

		 *                         - __insert_nid_to_list(FREE_NID_LIST)

		 */

		ne = __lookup_nat_cache(nm_i, nid);

		if (ne && (!get_nat_flag(ne, IS_CHECKPOINTED) ||

				nat_get_blkaddr(ne) != NULL_ADDR))

			goto err_out;

		e = __lookup_free_nid_list(nm_i, nid);

		if (e) {

			if (e->state == NID_NEW)

				ret = true;

			goto err_out;

		}

	}

	ret = true;

	err = __insert_nid_to_list(sbi, i, FREE_NID_LIST, true);

err_out:

	spin_unlock(&nm_i->nid_list_lock);

	radix_tree_preload_end();

	if (err) {

err:

	if (err)

		kmem_cache_free(free_nid_slab, i);

		return true;

	}

	return true;

	return ret;

}

static void remove_free_nid(struct f2fs_sb_info *sbi, nid_t nid)