Add wrapper for Linux BLKFLSBUF ioctl

[mattmacy]

FreeBSD has no analog. Buffered block devices were removed a decade
plus ago.

Signed-off-by: Matt Macy mmacy@FreeBSD.org

Motivation and Context

Description

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 #9508 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9508      +/-   ##
==========================================
- Coverage   79.13%   79.09%   -0.04%     
==========================================
  Files         416      416              
  Lines      123640   123657      +17     
==========================================
- Hits        97840    97812      -28     
- Misses      25800    25845      +45

Flag	Coverage Δ
#kernel	79.69% <ø> (-0.04%)	⬇️
#user	67.05% <100%> (+0.29%)	⬆️

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