MFC r323002 (by emaste): zfs: do not advertise edonr which is not yet supported

MFC r323002 (by emaste): zfs: do not advertise edonr which is not yet supported

illumos 4185 ("add new cryptographic checksums to ZFS: SHA-512,
Skein, Edon-R") was intentionally merged only partially in r289422,
without adding support for skein, sha512 and edonr on FreeBSD.

Support for skein and sha512 was added later on, but edonr is still not
implemented in FreeBSD.

Prior to this commit zfs(8) correctly rejected edonr, but with an error
message that claimed support:

fk@r500 ~ $zfs set checksum=edonr tank
cannot set property for 'tank': 'checksum' must be one of 'on | off | fletcher2 | fletcher4 | sha256 | sha512 | skein | edonr'