Don't allow attach to a raidz child vdev

[don-brady]

Motivation and Context

An automated test in FreeBSD encountered the case where an attach is attempted to a raidz child (#15536). This is not allowed and should fail but it now succeeds.

Regression from #15022.

Description

One of the checks in spa_vdev_attach() was failing to detect this case. The parent vdev ops can be raidz in two different attach cases:

Case 1 (attaching to raidz)

oldvd = pvd -> (raidz)
                / | \
               /  |  \
              /   |   \
            (c1) (c2) (c3)

Case 2 (attaching to a child of raidz)

     pvd -> (raidz)
             / | \
            /  |  \
           /   |   \
oldvd -> (c1) (c2) (c3)

Changed the check to only accept case 1

How Has This Been Tested?

ZTS functional/raidz
Ran reproducible test case from #15536 and it no longer happens

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)
• 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.

[ryao]

It might be helpful to clarify in the comment that the condition is a list of permitted cases under de morgan's law. If it were me, I would append the following:

As per De Morgan's law: !(A OR B OR C) === !A AND !B AND !C