In this patch, we change xattr block disk layout as below:

Before:
			xattr node block layout
+---------------------------------------------+---------------+-------------+
|           node block xattr entries          |   reserved    | node footer |
|                  4068 Bytes                 |    4 Bytes    |  24 Bytes   |

			In memory layout
+--------------------+---------------------------------+--------------------+
|    inline xattr    |     node block xattr entries    |      reserved      |
|     200 Bytes      |         4068 Bytes              |      4 Bytes       |

After:
			xattr node block layout
+-------------------------------------------------------------+-------------+
|                  node block xattr entries                   | node footer |
|                         4072 Bytes                          |  24 Bytes   |

			In memory layout
+--------------------+---------------------------------+--------------------+
|    inline xattr    |     node block xattr entries    |      reserved      |
|     200 Bytes      |         4072 Bytes              |      4 Bytes       |

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>