make zfs_async_block_max_blocks handle zero correctly

[TulsiJain]

Motivation and Context

Fixed this issue #8289

Description

frees all data when zfs_async_black_max_blocks is set to 0.

How Has This Been Tested?

build the load in ubuntu VM. set the zfs_async_black_max_blocks to 0 and everything worked as expected.

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.

[behlendorf]

Thanks for opening this, just a minor style nit.

[codecov]

Codecov Report

Merging #8829 into master will decrease coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #8829      +/-   ##
==========================================
- Coverage   78.99%   78.76%   -0.24%     
==========================================
  Files         382      382              
  Lines      117780   117813      +33     
==========================================
- Hits        93042    92794     -248     
- Misses      24738    25019     +281

Flag	Coverage Δ
#kernel	79.21% <100%> (-0.22%)	⬇️
#user	67.3% <100%> (-0.76%)	⬇️

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