nvpair: Check for un-terminated strings in packed nvlist

[tonyhutter]

Motivation and Context

Add additional nvlist checks.

Description

Add additional checks to verify a packed string or string array nvpair is terminated. Or more specifically, verify doing a strlen() on the prospective string does not overrun the packed nvlist buffer.

Also add additional checks in the libzfs_input_checks test case to verify un-terminated strings, and add in a nvlist ioctl payload fuzz test for good measure.

How Has This Been Tested?

Test case updated.

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, 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.