xfs: move directory block translatiosn to xfs_dir2_priv.h

Because they aren't actually part of the on-disk format, and so
shouldn't be in xfs_da_format.h.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>