raidz_test: Restore rand_data protection

It feels dirty to modify protection of a memory allocated via libc,
but at least we should try to restore it before freeing.

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