kfileitem: linkDest prevent readlink error when file is not a symlink

linkDest is a public function it can be called from anywhere, don't assume it is called after symlink status check. Dolphin does this: https://invent.kde.org/search?search=linkDest&nav_source=navbar&project_id=2775&group_id=1561&search_code=true

This will cause unnecessary warnings and save some syscalls. https://discuss.kde.org/t/dolphin-warnings-in-system-logs/22819

Testing: launch dolphin in terminal with: QT_LOGGING_RULES="kf.kio.core.warning=true" dolphin Use details view Show destination link column

After the MR there shouldn't be kf.kio.core: readlink failed! " errors output.