zdb: fix overflow of time estimation

The calculation of estimated time remaining in zdb -cc could overflow,
as reported in #10666. This patch fixes this, by using uint64_t instead
of ints in the calculations.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Teodor Spæren <teodor@sparen.no>
Closes #10666
Closes #12610