xfs: kill struct xfs_dir2_data

Remove the confusing xfs_dir2_data structure.  It is supposed to describe
an XFS dir2 data btree block, but due to the variable sized nature of
almost all elements in it it can't actuall do anything close to that
job.  In addition to accessing the fixed offset header structure it was
only used to get a pointer to the first dir or unused entry after it,
which can be trivially replaced by pointer arithmetics on the header
pointer.  For most users that is actually more natural anyway, as they
don't use a typed pointer but rather a character pointer for further
arithmetics.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>