xfs: make dir2 ftype offset pointers explicit

Rather than hiding the ftype field size accounting inside the dirent
padding for the ".." and first entry offset functions for v2
directory formats, add explicit functions that calculate it
correctly.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>