xfs: make Directory Iterator class an abstract class

Directory Iterator class represents one instance which could be Short, Extent, Leaf, Node or B+Tree, a good case for making it an abstract class

Change-Id: I925255caf4c4f8bc01a975740ef2ebf0bb2e1b49
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5764
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

xfs: Read Block directories

Block directories can now be read.
Change-Id: I9aa898638e6ac39cb903f2dd58cd184785e1a06b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2992
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

xfs: Read Short Form Directories

We can now read short form directories.
Change-Id: I23eb6ef52e8bb07643213e603ed0185f536c649f
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2952
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

xfs: An attempt at reading shortform dir

This patch gets read_dir, lookup and few others to work. Entries cannot
be listed yet except the root.

Change-Id: Ic4b7b61a794a1b75849e45777a383b3315b15aa8
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2915
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>