OpenZFS 8906 - uts: illumos rootfs should support salted cksum

[behlendorf]

Description

Porting notes:

• As of grub-2.02 these checksums are not supported. However, as
  pointed out in 'edonr' and 'skein' zpool features are not supported for root pools even though UEFI is a thing #6501 there are alternates see the issue for
  addition discussion.

Motivation and Context

OpenZFS-issue: https://illumos.org/issues/8906
OpenZFS-commit: openzfs/openzfs@7dec52f
Issue #6501

How Has This Been Tested?

Locally built, untested.

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)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.
• Change has been approved by a ZFS on Linux member.

[bunder2015]

I really think we should leave a note in the man pages along the lines of "grub still can't boot these pools", and/or that "EFI without grub is the only known working configuration".

edit: or perhaps "with a boot method capable of reading the root dataset or able to load the kernel without reading the root dataset"

[behlendorf]

Refreshed to include a warning about the GRUB incompatibly in the checksum property section.

[codecov]

Codecov Report

Merging #7714 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7714      +/-   ##
==========================================
+ Coverage   78.28%   78.28%   +<.01%     
==========================================
  Files         368      368              
  Lines      111981   111975       -6     
==========================================
- Hits        87665    87664       -1     
+ Misses      24316    24311       -5

Flag	Coverage Δ
#kernel	78.85% <ø> (+0.05%)	⬆️
#user	67.48% <ø> (+0.01%)	⬆️

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