uma: Fix a few problems with KASAN integration

uma: Fix a few problems with KASAN integration

- Ensure that all items returned by UMA are aligned to
  KASAN_SHADOW_SCALE (8).  This was true in practice since smaller
  alignments are not used by any consumers, but we should enforce it
  anyway.
- Use a non-zero code for marking redzones that appear naturally in
  items that are not a multiple of the scale factor in size.  Currently
  we do not modify keg layouts to force the creation of redzones.
- Use a non-zero code for marking freed per-CPU items, otherwise
  accesses of freed per-CPU items are not detected by the runtime.

Sponsored by:	The FreeBSD Foundation