zcp: get_prop: fix encryptionroot and encryption

[snajpa]

Motivation and Context

#12337

Description

It was reported that channel programs' zfs.get_prop doesn't work for
dataset properties encryption and encryptionroot.

They are handled in get_special_prop due to the need to call
dsl_dataset_crypt_stats to load those dsl props.

@grahamc already worked on a testcase for this (#12335), we could revive and merge that PR now

How Has This Been Tested?

Reproducer in #12337

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.