dtrace: Add some more annotations for KMSAN

• Don't allow FBT and kinst to instrument the KMSAN runtime.
• When fetching data from the traced thread's stack, mark it as initialized. It may well be uninitialized, but as dtrace permits arbitrary inspection of kernel memory, it isn't very useful to raise KMSAN reports.
• Mark data copied in from userspace as initialized, as we do for copyin() etc. using interceptors.

MFC after: 2 weeks