tests/zdb_encrypted: parse numbers a little more robustly

On FreeBSD, wc prints some leading spaces, while on Linux it does not.
So we tell ksh to expect an integer, and it does the rest.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #14791
Closes #14797