btrfs: init ref_index to zero in add_inode_ref

	int log_ref_ver = 0;

	u64 parent_objectid;

	u64 inode_objectid;

	u64 ref_index;

	u64 ref_index = 0;

	int ref_struct_size;

	ref_ptr = btrfs_item_ptr_offset(eb, slot);