quota: widen timestamps for the fs_disk_quota structure

Soon, XFS will support quota grace period expiration timestamps beyond
the year 2038, widen the timestamp fields to handle the extra time bits.
Internally, XFS now stores unsigned 34-bit quantities, so the extra 8
bits here should work fine.  (Note that XFS is the only user of this
structure.)

Link: https://lore.kernel.org/r/20200909163413.GJ7955@magnolia
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>