xfs: cleanup xfs_dir2_data_freescan_int

Use an offset as the main means for iteration, and only do pointer
arithmetics to find the data/unused entries.

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>