xfsdocs: move dir-attr btree sections to a separate chapter

Instead of scattering dabtree documentation across two sections underneath the directory chapter, just give them a separate chapter.

---
 design/XFS_Filesystem_Structure/btrees.asciidoc    |    2 
 design/XFS_Filesystem_Structure/dabtrees.asciidoc  |  219 ++++++++++++++++++++
 .../XFS_Filesystem_Structure/directories.asciidoc  |  157 --------------
 .../xfs_filesystem_structure.asciidoc              |    2 
 4 files changed, 223 insertions(+), 157 deletions(-)
 create mode 100644 design/XFS_Filesystem_Structure/dabtrees.asciidoc

index 306e061..91a53c4 100644
@@ -1,4 +1,4 @@
-= B+trees
+= Fixed Length Record B+trees
 
 XFS uses b+trees to index all metadata records.  This well known data structure
 is used to provide efficient random and sequential access to metadata records

index 4b0a054..5dba1c7 100644
@@ -68,6 +68,8 @@ Global Structures
 
 include::btrees.asciidoc[]
 
+include::dabtrees.asciidoc[]
+
 include::allocation_groups.asciidoc[]
 
 include::rmapbt.asciidoc[]

Splendid idea. Make it so :)

Dave Chinner
[email protected]