Fix integer overflow in get_next_chunk()

dn->dn_datablksz type is uint32_t and need to be casted to uint64_t
to avoid an overflow when the record size is greater than 4 MiB.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olivier Mazouffre <olivier.mazouffre@ims-bordeaux.fr>
Closes #8778
Closes #8797