xfs: replace xfs_dir3_data_endp with xfs_dir3_data_end_offset

All the callers really want an offset into the buffer, so adopt
the helper to return that instead.

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>