xfs: redefine xfs_timestamp_t

Redefine xfs_timestamp_t as a __be64 typedef in preparation for the
bigtime functionality.  Preserve the legacy structure format so that we
can let the compiler take care of masking and shifting.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Gao Xiang <hsiangkao@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>