ZTS: fix reservation_013_pos integer overflow

When using large disks the integers for calculating sizes can
overflow past 2**31. Changing to long integers with typeset
should correct this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: bunder2015 <omfgbunder@gmail.com>
Closes #4444
Closes #7451