Better user experience for errata 4

[tcaputi]

This patch attempts to address some user concerns that have arisen
since errata 4 was introduced.

• The errata warning has been made less scary for users without
  any encrypted datasets.

• The errata warning now clears itself without a pool reimport if
  the bookmark_v2 feature is enabled and no encrypted datasets
  exist.

• It is no longer possible to create new encrypted datasets without
  enabling the bookmark_v2 feature, thus helping to ensure that the
  errata is resolved.

Signed-off-by: Tom Caputi [email protected]

Motivation and Context

#8503

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.

[codecov]

Codecov Report

Merging #8504 into master will decrease coverage by 0.15%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master    #8504      +/-   ##
==========================================
- Coverage   78.67%   78.51%   -0.16%     
==========================================
  Files         380      380              
  Lines      116230   116238       +8     
==========================================
- Hits        91440    91270     -170     
- Misses      24790    24968     +178

Flag	Coverage Δ
#kernel	79.11% <62.5%> (+0.09%)	⬆️
#user	67.01% <33.33%> (-0.45%)	⬇️

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