xfsdocs: fix extent record format image

xfsdocs: fix extent record format image

The image of 'design/XFS_Filesystem_Structure/images/31.png' depicts
extent record format as below:

+----+-------------------------+---------------------+------------+
|flag|bits 72 to 126(54)       |bits 21 to 72(52)    |bit 0-20(21)|
|    |logical file block offset|absolute block number|# blocks    |
+----+-------------------------+---------------------+------------+

There has a typo for 'logical file block offset' part, it writes 'bits
72 to 126', but it should be 'bits 73 to 126' because the 72st bit is
consumed by 'absolute block number' part, this patch fix this problem
and redrawing the image as below:

+---------+-------------------------+---------------------+----------+
|bits[127]|bits[73-126]             |bits[21-72]          |bits[0-20]|
+---------+-------------------------+---------------------+----------+
|flag     |logical file block offset|absolute block number|# blocks  |
+---------+-------------------------+---------------------+----------+

Signed-off-by: Xiaole He <hexiaole@kylinos.cn>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: convert the raw table into asciidoc and remove the png]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>