btrfs: batch up release of reserved metadata for delayed items used for deletion

With Filipe's recent rework of the delayed inode code one aspect which
isn't batched is the release of the reserved metadata of delayed inode's
delete items. With this patch on top of Filipe's rework and running the
same test as provided in the description of a patch titled
"btrfs: improve batch deletion of delayed dir index items" I observe
the following change of the number of calls to btrfs_block_rsv_release:

Before this change:
- block_rsv_release:                      1004
- btrfs_delete_delayed_items_total_time: 14602
- delete_batches:                          505

After:
- block_rsv_release:                       510
- btrfs_delete_delayed_items_total_time: 13643
- delete_batches:                          507
Reviewed-by: NFilipe Manana <fdmanana@suse.com>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/delayed-inode.c

@@ -864,11 +864,13 @@ static int btrfs_batch_delete_items(struct btrfs_trans_handle *trans,
 				    struct btrfs_path *path,
 				    struct btrfs_delayed_item *item)
 {
+	struct btrfs_fs_info *fs_info = root->fs_info;
 	struct btrfs_delayed_item *curr, *next;
 	struct extent_buffer *leaf = path->nodes[0];
 	LIST_HEAD(batch_list);
 	int nitems, slot, last_slot;
 	int ret;
+	u64 total_reserved_size = item->bytes_reserved;
 
 	ASSERT(leaf != NULL);
 
@@ -905,14 +907,27 @@ static int btrfs_batch_delete_items(struct btrfs_trans_handle *trans,
 		nitems++;
 		curr = next;
 		list_add_tail(&curr->tree_list, &batch_list);
+		total_reserved_size += curr->bytes_reserved;
 	}
 
 	ret = btrfs_del_items(trans, root, path, path->slots[0], nitems);
 	if (ret)
 		return ret;
 
+	/* In case of BTRFS_FS_LOG_RECOVERING items won't have reserved space */
+	if (total_reserved_size > 0) {
+		/*
+		 * Check btrfs_delayed_item_reserve_metadata() to see why we
+		 * don't need to release/reserve qgroup space.
+		 */
+		trace_btrfs_space_reservation(fs_info, "delayed_item",
+					      item->key.objectid, total_reserved_size,
+					      0);
+		btrfs_block_rsv_release(fs_info, &fs_info->delayed_block_rsv,
+					total_reserved_size, NULL);
+	}
+
 	list_for_each_entry_safe(curr, next, &batch_list, tree_list) {
-		btrfs_delayed_item_release_metadata(root, curr);
 		list_del(&curr->tree_list);
 		btrfs_release_delayed_item(curr);
 	}