kassert: Turn 'poisoned_buf' into a 'void *'

This is a step to make <sys/kassert.h> usable without the need to
explicitly include other headers.

'poisoned_buf' is only used for its value and never subject to pointer
arithmetics. Keeping 'caddr_t' would require <sys/kassert.h> to include
<sys/types.h> (at least on INVARIANTS).

Reviewed by: markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49971