Note: this commit slightly changes zfs_ioc_unload_key() ABI. This allow
to differentiate the case where we tried to unload a key on a
non-existing dataset (ENOENT) from the situation where a dataset has
no key loaded: this is consistent with the "change" case where trying
to zfs_ioc_change_key() from a dataset with no key results in EACCES.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7230