xfs: move xfs_ondisk.h to libxfs/

Move xfs_ondisk.h to libxfs so that we can do the struct sanity checks
in userspace libxfs as well.  This should allow us to retire the
somewhat fragile xfs/122 test on xfstests.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>