Change zpool offline spares policy

[tonyhutter]

Motivation and Context

Make sure you can't force fault an offline spare.

Description

The zpool offline man page says that you cannot use zpool offline on spares. However, testing found that you could in fact force fault (zpool offline -f) spares.

Change the policy to:

1. You can never force-fault or offline dRAID spares.
2. You can only force-fault or offline traditional spares if they're active.

How Has This Been Tested?

Added test case

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[akashb-22]

LGTM.

[tonyhutter]

I've tweaked this PR a little. The new policy for force-faulting spares is:

1. You can never force-fault or offline dRAID spares.
2. You can only force-fault or offline traditional spares if they're active.