xfs: kill xfs_dir2_sf_off_t

Just use an array of two unsigned chars directly to avoid problems
with architectures that pad the size of structures.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>