network: fix IPv6PrivacyExtensions=kernel handling

When set to "kernel", systemd is not supposed to touch that sysctl.

5e0534f1, part of
https://github.com/systemd/systemd/pull/17240 forgot to handle that
case.

Fixes https://github.com/systemd/systemd/issues/18003

(cherry picked from commit d3ccb1b4)