GCC 15: Fix unterminated-string-initialization (#17244)

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 don't need to be
NULL terminated, but we do so to make GCC happy.

Closes: #17242

Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>

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

@@ -61,7 +61,7 @@ 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;