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 <[email protected]>
Reviewed-by: Richard Laager <[email protected]>
Co-authored-by: Didier Roche <[email protected]>
Signed-off-by: Didier Roche <[email protected]>
Closes #10477

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