OpenZFS 8906 - uts: illumos rootfs should support salted cksum

Porting notes:
* As of grub-2.02 these checksums are not supported.  However, as
  pointed out in #6501 there are alternatives such as EFISTUB which
  work and have no such restriction.  A warning was added to the
  checksum property section of the zfs.8 man page.

Authored by: Toomas Soome <tsoome@me.com>
Reviewed by: C Fraire <cfraire@me.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Yuri Pankov <yuripv@yuripv.net>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://illumos.org/issues/8906
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/7dec52f
Closes #6501
Closes #7714

man/man8/zfs.8

@@ -29,7 +29,7 @@
 .\" Copyright 2016 Nexenta Systems, Inc.
 .\" Copyright 2016 Richard Laager. All rights reserved.
 .\"
-.Dd June 28, 2017
+.Dd July 13, 2018
 .Dt ZFS 8 SMM
 .Os Linux
 .Sh NAME
@@ -1049,6 +1049,9 @@ The
 and
 .Sy edonr
 checksum algorithms require enabling the appropriate features on the pool.
+These algorithms are not supported by GRUB and should not be set on the
+.Sy bootfs
+filesystem when using GRUB to boot the system.
 Please see
 .Xr zpool-features 5
 for more information on these algorithms.