unit/fletcher: test fletcher-2 and fletcher-4 checksums

[chrislongros]

Motivation and Context

Fletcher algorithm is the is checksum and hashing mechanism used in ZFS for error detection and data integrity.

Fletcher checksums have no test coverage. This adds a test_fletcher suite that explicitly tests the Fletcher-2 and Fletcher-4 routines in zfs_fletcher.c, including every SIMD and scalar implementation the host supports.

Description

Adds a test_fletcher suite calling the Fletcher checksum functions:

• Fletcher-4: known answer test (KAT), incremental vs single-call marching, fletcher_4_native_varsize() vs fletcher_4_native(), native vs byteswap, and equality across every supported implementation (selected by name with fletcher_4_impl_set()).
• Fletcher-2: known answer test (KAT), and incremental vs single-call matching.

How Has This Been Tested?

make unit T=fletcher:

> make unit T=fletcher
  UNITTEST tests/unit/test_fletcher
Running test suite with seed 0xc3bf5e76...
fletcher.fletcher4_known             [ OK    ] [ 0.00000138 / 0.00000048 CPU ]
fletcher.fletcher4_incremental       [ OK    ] [ 0.00000356 / 0.00000312 CPU ]
fletcher.fletcher4_varsize           [ OK    ] [ 0.00000453 / 0.00000409 CPU ]
fletcher.fletcher4_byteswap          [ OK    ] [ 0.00000360 / 0.00000319 CPU ]
fletcher.fletcher4_impls             [ OK    ] [ 0.00000892 / 0.00000890 CPU ]
fletcher.fletcher2_known             [ OK    ] [ 0.00000052 / 0.00000052 CPU ]
fletcher.fletcher2_incremental       [ OK    ] [ 0.00000235 / 0.00000234 CPU ]
7 of 7 (100%) tests successful, 0 (0%) test skipped.

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[chrislongros]

I am also considering a Monte Carlo test that tests bit flipping and random-sized increments.