Avoid Linux hung task message in ZTHR

[dweeezil]

Description

s/cv_wait/cv_timedwait/ in zthr_procedure().

Motivation and Context

Avoid triggering the Linux kernel "hung task" warning

How Has This Been Tested?

Ran the "removal" set of tests in ZTS.

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 #7441 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7441      +/-   ##
==========================================
- Coverage   76.59%   76.55%   -0.05%     
==========================================
  Files         335      335              
  Lines      107017   107018       +1     
==========================================
- Hits        81971    81925      -46     
- Misses      25046    25093      +47

Flag	Coverage Δ
#kernel	77.04% <100%> (+0.14%)	⬆️
#user	65.97% <100%> (-0.24%)	⬇️

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