vfs: Add KASAN state transitions for vnodes

vnodes are a bit special in that they may exist on per-CPU lists even
while free. Add a KASAN-only destructor that poisons regions of each
vnode that are not expected to be accessed after a free.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29459