xfs: extended attributes necessary hooks and shortform attributes support

- Implemented all necessary hooks for extended attributes in kernel_interface.cpp

- Implemented an abstract class attribute which will act as an interface between all forms of extended attributes in xfs

- An attempt to read shortform extended attributes

Change-Id: Icff9329b790f9fcfc0b078f61059795af9d9e486
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5570
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>