GCC 15: Fix unterminated-string-initialization

Fix build errors on Fedora 42 like:

  module/zcommon/zfs_valstr.c:193:16: error: initializer-string for
  array of 'char' truncates NUL terminator but destination lacks
  'nonstring' attribute (3 chars into 2 available)

The arrays in zpool_vdev_os.c and zfs_valstr.c are not meant to be
NULL terminated.

Closes: openzfs#17242
Signed-off-by: Tony Hutter <[email protected]>

Author: tonyhutter

cmd/zpool/os/linux/zpool_vdev_os.c

@@ -88,7 +88,7 @@
 
 typedef struct vdev_disk_db_entry
 {
-	char id[24];
+	char id[24]	__attribute__((nonstring));
 	int sector_size;
 } vdev_disk_db_entry_t;
 

module/zcommon/zfs_valstr.c

@@ -39,7 +39,7 @@
  */
 typedef struct {
 	const char	vb_bit;
-	const char	vb_pair[2];
+	const char	vb_pair[2]	 __attribute__((nonstring));
 	const char	*vb_name;
 } valstr_bit_t;
 

tests/zfs-tests/cmd/file/largest_file.c

@@ -62,7 +62,7 @@ main(int argc, char **argv)
 	offset_t	llseek_ret = 0;
 	int		write_ret = 0;
 	int		err = 0;
-	char		mybuf[5] = "aaaa\0";
+	char		mybuf[5] = "aaaa";
 	char		*testfile;
 	mode_t		mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
 	struct sigaction sa;