Revert "Handle zap_add() failures in mixed ... "

[tonyhutter]

Description

This reverts commit cc63068. That commit caused disappearing files in: #7401

Motivation and Context

Closes: #7401

How Has This Been Tested?

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.

[codecov]

Codecov Report

Merging #7416 into master will decrease coverage by 0.13%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #7416      +/-   ##
==========================================
- Coverage   76.31%   76.17%   -0.14%     
==========================================
  Files         330      330              
  Lines      104279   104235      -44     
==========================================
- Hits        79583    79406     -177     
- Misses      24696    24829     +133

Flag	Coverage Δ
#kernel	76.39% <90%> (+0.52%)	⬆️
#user	65.25% <100%> (-0.38%)	⬇️

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