zcp: get_prop: fix encryptionroot and encryption

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.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>
Co-authored-by: Graham Christensen <graham@grahamc.com>
Closes #17280