kmem: Add KASAN state transitions

Memory allocated with kmem_* is unmapped upon free, so KASAN doesn't
provide a lot of benefit, but since allocations are always a multiple of
the page size we can create a redzone when the allocation request size
is not a multiple of the page size.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29458