libxfs: create a real struct timespec64

libxfs: create a real struct timespec64

Create a real struct timespec64 that supports 64-bit seconds counts.
The C library struct timespec doesn't support this on 32-bit
architectures and we cannot lose the upper bits in the incore inode.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>