Make unloading the key more robust

Make unloading the key more robust

The unit was failing instead of stopping if someone manually unloaded
the key before stopping the unit (zfs unload-key is failing on an
unavailable key).
Follow a similar logic than for loading the key, checking for the key
status before unloading it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Co-authored-by: Didier Roche <didrocks@ubuntu.com>
Signed-off-by: Didier Roche <didrocks@ubuntu.com>
Closes #10477