JSON: fix user properties output for zfs list

[usaleem-ix]

Motivation and Context

zfs list -o <user_property> does not output user properties

Description

This commit fixes JSON output for zfs list when user properties are requested with -o flag. This case needed to be handled specifically since zfs_prop_to_name does not return property name for user properties, instead it is stored in pl->pl_user_prop.

How Has This Been Tested?

Manually tested by setting the user properties and retrieving them in JSON format.

Types of changes

• [x] 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:

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

[behlendorf]

But seems like we have some problem with user properties also in zpool list -o, and not only with JSON.

I'll go ahead and merge this fix and we can resolve any other user properties issues with a new PR.