fix for issue #12001

[gamanakis]

Motivation and Context

Closes #12001

Description

How Has This Been Tested?

Manually, using the scripts provided in #12014

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.

[gamanakis]

Kindly pinging @robn. Following your comments at #12014, I believe this commit is correct. Which would explain #12001.

[gamanakis]

Turns out I was wrong. Through it made sense that if a slot in a multi node exists, we should txg_wait_synced() to free it.

[gamanakis]

Wait I missed something I think