Don't allow dnode allocation if dn_holds != 0

[tcaputi]

This patch simply fixes a small bug where dnode_hold_impl() could
attempt to allocate a dnode that was in the process of being freed,
but which still had active references. This patch simply adds the
required check.

Signed-off-by: Tom Caputi tcaputi@datto.com

How Has This Been Tested?

This problem was found and the fix was confirmed using ztest.

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 #8249 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #8249      +/-   ##
==========================================
- Coverage   78.56%   78.51%   -0.05%     
==========================================
  Files         379      379              
  Lines      114924   114899      -25     
==========================================
- Hits        90286    90215      -71     
- Misses      24638    24684      +46

Flag	Coverage Δ
#kernel	79.01% <100%> (+0.17%)	⬆️
#user	67.32% <100%> (-0.36%)	⬇️

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