uma: Add KASAN state transitions

[markj]

Summary

- Add a UMA_ZONE_NOKASAN flag to indicate that items from a particular
zone should not be sanitized. This is applied implicitly for NOFREE
and cache zones.
- Add KASAN call backs which get invoked:
1) when a slab is imported into a keg
2) when an item is allocated from a zone
3) when an item is freed to a zone
4) when a slab is freed back to the VM

In state transitions 1 and 3, memory is poisoned so that accesses will
trigger a panic. In state transitions 2 and 4, memory is marked
valid.
- Disable trashing if KASAN is enabled. It just adds extra CPU overhead
to catch problems that are detected by KASAN.

[emaste]

sys/vm/uma.h

279

Aside, in cases like this I'd like style(9) to put the first line of the comment beside the /*.

sys/vm/uma_core.c

536

Sort of bikeshed territory, but I wonder if

#ifdef KASAN
... implementation
#else
... stubs
#endif

isn't more straightforward

1688–1690

these and a couple of other changes can be committed independently first?

[markj]

sys/vm/uma_core.c

1688–1690

I should just drop this part, it's left over from an earlier version of the patch.

[markj]

• Drop spurious hunk.
• Provide separate stubs for the !KASAN case.