xfs: reshuffle dir2 definitions around for userspace

Many of the definitions within xfs_dir2_priv.h are needed in
userspace outside libxfs. Definitions within xfs_dir2_priv.h are
wholly contained within libxfs, so we need to shuffle some of the
definitions around to keep consistency across files shared between
user and kernel space.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>