xfs: use uuid_copy() helper to abstract uuid_t

uuid_t definition is about to change.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
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>