Prefer VERIFY0 over comparision with 0 or NULL

[robn]

Motivation and Context

VERIFY0(n) gives better output than VERIFY(n == 0) because it'll show both values, and its shorter to type than VERIFY3U(n, == 0). So let's do that, and same for ASSERT and pointer variants too,

Description

• Use VERIFY0(n) instead of VERIFY(n == 0), VERIFY3U(n, ==, 0), VERIFY3S(n, ==, 0)
• Use ASSERT0(n) instead of ASSERTn == 0), ASSERT3U(n, ==, 0), ASSERT3S(n, ==, 0)
• Use VERIFY0P(n) instead of VERIFY(n == NULL), VERIFY3P(n, ==, NULL)
• Use ASSERT0P(n) instead of ASSERT(n == NULL), ASSERT3P(n, ==, NULL)

How Has This Been Tested?

Compile checked on Linux and FreeBSD, with and without --enable-debug. ZTS run completed on Linux with --enable-debug.

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.

[amotin]

Thanks. This partially overlaps with old #14424. I tend to close it now, but wonder if you wish to take anything out of it, even if only ideas.

[robn]

Yeah, I'm headed in the same direction as those. Nothing more for this PR; its the sort of thing I just do when I have a free evening and don't want to think too hard :)

[behlendorf]

Nice. If you can resolve the latest conflicts I'll get this bit of cleanup merged.

[robn]

Rebased!