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>