zcp: implement zfs.get_prop() for bookmarks

[delan]

this patch is still pretty rough, but i have so many questions to ask! see comments marked XXX :)

Motivation and Context

bookmarks support a small set of dataset properties that either reflect the snapshot they were based on (guid, createtxg, creation, ivsetguid) or are unique to the redaction bookmark (redact_snaps, redact_complete).

zfs get <property> can query bookmark properties, but zfs.get_prop() is unable to query bookmark properties (#17949), because doing so for bookmarks requires a different procedure to other datasets.

Description

this patch fixes #17949 by adding a separate code path for bookmarks in zcp_get_prop(), which effectively calls dsl_bookmark_fetch_props() instead of whatever we do for other datasets.

How Has This Been Tested?

(TODO: see issue comments)

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

[delan]

i think this was because in C, 0 || 2 is 1, not 2