raidz_test: Set io_offset reasonably
- io_offset of 1 makes no sense. Set default to 0.
- Initialize io_offset in all cases.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17977