GCC 15: Fix unterminated-string-initialization

[tonyhutter]

Motivation and Context

Build ZFS on Fedora 42 (GCC 15). Closes: #17242

Description

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)

GCC 15 is flagging arrays that look like they're strings, but have no space for the ending NULL terminator. The arrays in zpool_vdev_os.c and zfs_valstr.c look like strings but are not meant to be NULL terminated, which is why they are getting flagged.

How Has This Been Tested?

Successfully built on Fedora 42.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[tonyhutter]

I ended up ditching __attribute__((nonstring)) and just expanded the arrays. CI is happy now.