config: Fix LLVM-21 -Wuninitialized-const-pointer warning

[behlendorf]

Motivation and Context

#17682

Description

By default LLVM-21 enables -Wuninitialized-const-pointer which results in the following compiler warning and the bdev_file_open_by_path() interface not being detected for 6.9 and newer kernels. The blk_holder_ops are not used by the ZFS code so we can safely use a NULL argument for this check.

bdev_file_open_by_path/bdev_file_open_by_path.c:110:54: error: variable 'h' is uninitialized when passed as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]

Note: Initially I was inclined to initialize the blk_holder_ops struct which is the approach we use elsewhere, but after discovering how many times that structure has changed I opted for the simpler more robust change to pass NULL.

How Has This Been Tested?

Locally compiled, but not on all of the relevant kernels. The CI can help out some there, but we'll also want to do some manual testing with the relevant llvm versions.

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.

[robn]

Yeah, this looks good to me. We are of course going to let a signature change through, but the actual build should catch that, and if not, CI. And if that gets through, maybe we have other life choices to consider.