config: set `-Wno-format-truncation` globally

[robn]

Motivation and Context

-Wformat-truncation looks for places where the return code of snprintf() is unchecked and the provided buffer might be too short. This is based on a heuristic that can change between compiler versions.

It has been seen to get this wrong in ddt_object_name(), leading to DDT_NAMELEN being increased somewhat arbitrarily.

There's no good reason to have this warning enabled, so here we disable it everywhere. Truncation may be undesirable, but snprintf() is guaranteed to emit a trailing null, so at worst we get a short string, not a buffer overrun.

Closes #15905.

Description

Add NO_FORMAT_TRUNCATION to AM_CFLAGS globally.

Reduce DDT_NAMELEN to something sensible, since we're no longer blowing up on it.

How Has This Been Tested?

Compiles locally, quick dedup check looks right.

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)
• [x] Code cleanup (non-breaking change which makes code smaller or more readable)
• [ ] 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:

• [x] My code follows the OpenZFS code style requirements.
• [ ] I have updated the documentation accordingly.
• [x] 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.
• [x] All commit messages are properly formatted and contain Signed-off-by.