zdb -vvvvv on ztest pool dies with "out of memory"

zdb -vvvvv on ztest pool dies with "out of memory"

ztest creates some extremely large files as part of its
operation. When zdb tries to dump a large enough file, it
can run out of memory or spend an extremely long time
attempting to print millions or billions of uint64_ts.

We cap the amount of data from a uint64 object that we
are willing to read and print.

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
External-issue: DLPX-53814
Closes #8947