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

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.  Initialize the structure in
the configure test to resolve the warning and detect the interface.

    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]

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

Author: behlendorf

config/kernel-blkdev.m4

@@ -24,14 +24,22 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BLKDEV_GET_BY_PATH_4ARG], [
 	ZFS_LINUX_TEST_SRC([blkdev_get_by_path_4arg], [
 		#include <linux/fs.h>
 		#include <linux/blkdev.h>
+
+		static void
+		test_mark_dead(struct block_device *bdev) {
+			return;
+		}
+
+		const struct blk_holder_ops test_holder_ops = {
+			.mark_dead = test_mark_dead,
+		};
 	], [
 		struct block_device *bdev __attribute__ ((unused)) = NULL;
 		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, &test_holder_ops);
 	])
 ])
 
@@ -43,14 +51,22 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BLKDEV_BDEV_OPEN_BY_PATH], [
 	ZFS_LINUX_TEST_SRC([bdev_open_by_path], [
 		#include <linux/fs.h>
 		#include <linux/blkdev.h>
+
+		static void
+		test_mark_dead(struct block_device *bdev, bool flag) {
+			return;
+		}
+
+		const struct blk_holder_ops test_holder_ops = {
+			.mark_dead = test_mark_dead,
+		};
 	], [
 		struct bdev_handle *bdh __attribute__ ((unused)) = NULL;
 		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, &test_holder_ops);
 	])
 ])
 
@@ -63,14 +79,22 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BDEV_FILE_OPEN_BY_PATH], [
 	ZFS_LINUX_TEST_SRC([bdev_file_open_by_path], [
 		#include <linux/fs.h>
 		#include <linux/blkdev.h>
+
+		static void
+		test_mark_dead(struct block_device *bdev, bool flag) {
+			return;
+		}
+
+		const struct blk_holder_ops test_holder_ops = {
+			.mark_dead = test_mark_dead,
+		};
 	], [
 		struct file *file __attribute__ ((unused)) = NULL;
 		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, &test_holder_ops);
 	])
 ])