Backport #17684 to 2.3: config: Fix LLVM-21 -Wuninitialized-const-pointer warning

[alex-moch]

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]

Reviewed-by: Rob Norris [email protected]

Closes #17682
Closes #17684
(cherry picked from commit 9acedba)

Motivation and Context

Builds with LLVM/Clang 21 fail due to the new -Wuninitialized-const-pointer warning. This prevents proper detection of the bdev_file_open_by_path() interface on kernel 6.9+.

Fixes #17959 and #17994

Description

This cherry-pick fixes a build failure with LLVM/Clang 21 by initializing the blk_holder_ops pointer to NULL in the bdev_file_open_by_path() configure check. Since ZFS doesn't use blk_holder_ops, this change is safe and allows the configure check to pass.

How Has This Been Tested?

Clean cherry-pick from the master branch where it was already tested and reviewed.

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.