xfs: remove the data_dotdot_offset field in struct xfs_dir_ops

xfs: remove the data_dotdot_offset field in struct xfs_dir_ops

Source kernel commit: 2eb68a5d3619b80dec745f71df8af5f80cda16f8

The data_dotdot_offset value is always equal to data_entry_offset plus
the fixed size of the "." entry.  Right now calculating that fixed size
requires an indirect call, but by the end of this series it will be
an inline function that can be constant folded.

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>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>