With this change, we don't need to reserve additional space in node block,
just keep reserved space in logical in-memory layout. So that it would help
to enlarge valid free space of xattr node block.
As tested, generic/026 shows max stored xattr entires number increases from
531 to 532 when inline_xattr option is enabled.
Signed-off-by: Chao Yu <yuchao0@huawei.com> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>