xfs: use uuid_be to implement the uuid_t type

Use the generic Linux definition to implement our UUID type, this will
allow using more generic infrastructure in the future.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>