ZTS: Clarify loop variable name in zfs copies test

[ghost]

Signed-off-by: Ryan Moeller ryan@iXsystems.com

Motivation and Context

The loop variable name "val" is ambiguous.

Description

do_vol_test takes the number of copies as the second parameter, so name the loop variable copies instead of val.

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.

[codecov]

Codecov Report

Merging #9445 into master will increase coverage by 11.16%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #9445       +/-   ##
===========================================
+ Coverage      68%   79.17%   +11.16%     
===========================================
  Files         365      412       +47     
  Lines      117487   123602     +6115     
===========================================
+ Hits        79895    97857    +17962     
+ Misses      37592    25745    -11847

Flag	Coverage Δ
#kernel	79.73% <ø> (+12.57%)	⬆️
#user	67.04% <ø> (+6.94%)	⬆️

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