xfs: remove the ->data_unused_p method

Replace the two users of the ->data_unused_p dir ops method with a
direct calculation using ->data_entry_offset, and clean them up a bit.
xfs_dir2_sf_to_block already had an offset variable containing the
value of ->data_entry_offset, which we are now reusing to make it
clear that the initial freespace entry is at the same place that
we later fill in the 1 entry, and in xfs_dir3_data_init the function
is cleaned up a bit to keep the initialization of fields of a given
structure close to each other, and to avoid a local variable.

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>