xfs: redefine xfs_ictimestamp_t

Redefine xfs_ictimestamp_t as a uint64_t typedef in preparation for the
bigtime functionality.  Preserve the legacy structure format so that we
can let the compiler take care of the 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>