MFC r323002: zfs: do not advertise unsupported hash algorithms

MFC r323002: zfs: do not advertise unsupported hash algorithms

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 (in head), but none of
these are supported in stable/10. Prior to this commit zfs(8) correctly
rejected these algorithms, 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'

(This commit removes sha512 and skein in addition to edonr from the
merge of head's r323002.)

PR:		204055
Submitted by:	Fabian Keil
Approved by:	re (kib)
Obtained from:	ElectroBSD

git-svn-id: svn://svn.freebsd.org/base/stable/10@323331 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f