nvpair: i_get_value_size() string array handling tweak

[ryao]

Motivation and Context

The strnlen() function needs to be given the length of the remaining
region to behave as intended, but it was given the length of the total
region on packed strings.

Description

We tighten max_size on packed strings, so that strnlen() works as expected.

Grok 4.5 Build Beta found this bug.

How Has This Been Tested?

The buildbot can test it.

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair 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.