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

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 <robn@despairlabs.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#17682
Closes openzfs#17684

Author: behlendorf

config/kernel-blkdev.m4

@@ -29,9 +29,8 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BLKDEV_GET_BY_PATH_4ARG], [
 		const char *path = "path";
 		fmode_t mode = 0;
 		void *holder = NULL;
-		struct blk_holder_ops h;
 
-		bdev = blkdev_get_by_path(path, mode, holder, &h);
+		bdev = blkdev_get_by_path(path, mode, holder, NULL);
 	])
 ])
 
@@ -48,9 +47,8 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BLKDEV_BDEV_OPEN_BY_PATH], [
 		const char *path = "path";
 		fmode_t mode = 0;
 		void *holder = NULL;
-		struct blk_holder_ops h;
 
-		bdh = bdev_open_by_path(path, mode, holder, &h);
+		bdh = bdev_open_by_path(path, mode, holder, NULL);
 	])
 ])
 
@@ -68,9 +66,8 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BDEV_FILE_OPEN_BY_PATH], [
 		const char *path = "path";
 		fmode_t mode = 0;
 		void *holder = NULL;
-		struct blk_holder_ops h;
 
-		file = bdev_file_open_by_path(path, mode, holder, &h);
+		file = bdev_file_open_by_path(path, mode, holder, NULL);
 	])
 ])