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>
diff --git a/design/XFS_Filesystem_Structure/data_extents.asciidoc b/design/XFS_Filesystem_Structure/data_extents.asciidoc
index 4f1109b..29c78fe 100644
--- a/design/XFS_Filesystem_Structure/data_extents.asciidoc
+++ b/design/XFS_Filesystem_Structure/data_extents.asciidoc
@@ -17,7 +17,19 @@
 An extent is 128 bits in size and uses the following packed layout:
 .Extent record format
+|logical file block offset
+|absolute block number
+|# of blocks
 The extent is represented by the +xfs_bmbt_rec+ structure which uses a big
 endian format on-disk. In-core management of extents use the +xfs_bmbt_irec+
diff --git a/design/XFS_Filesystem_Structure/images/31.png b/design/XFS_Filesystem_Structure/images/31.png
deleted file mode 100644
index 48b0172..0000000
--- a/design/XFS_Filesystem_Structure/images/31.png
+++ /dev/null
Binary files differ