Fix spa_nonallocating_dspace for special vdevs

[amotin]

Since spa_dspace accounts only normal allocation class space, spa_nonallocating_dspace should do the same. Otherwise we may get negative overflow or respective assertion in spa_update_dspace() if removed special/dedup vdev is bigger than all normal class space.

How Has This Been Tested?

Originally I spotted it while reviewing ztest CI failures. After that I reproduced it manually by trying to remove from a pool special vdev that is bigger than normal vdev. Applying the patch fixed the manual test.

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.

[jumbi77]

Well spotted. Is a dedicated test case useful here?

[amotin]

@jumbi77 I haven't looked what device removal tests do we actually have in ZTS now, but if you wish to review them and add a case to something existing -- be my guest.