Update maximum kernel version to 5.4

[behlendorf]

Motivation and Context

Issue #9754, updated to accurately report the current maximum
supported kernel version.

Description

Increase the maximum supported kernel version to 5.4. This was
verified using the Fedora 5.4.2-300.fc31.x86_64 kernel.

How Has This Been Tested?

Verified by compiling ZFS using the Fedora 5.4.2-300.fc31.x86_64
kernel, loading the modules, and running a subset of the 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.

[codecov]

Codecov Report

Merging #9759 into master will decrease coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #9759    +/-   ##
========================================
- Coverage      80%      80%   -<1%     
========================================
  Files         385      385            
  Lines      121470   121470            
========================================
- Hits        96706    96672    -34     
- Misses      24764    24798    +34

Flag	Coverage Δ
#kernel	80% <ø> (ø)	⬆️
#user	67% <ø> (ø)	⬇️

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