xfs: remove the data_dot_offset field in struct xfs_dir_ops

The data_dot_offset value is always equal to data_entry_offset given
that "." is always the first entry in the directory.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>