mempool: do not use ksize() for poisoning

Nothing appears to be using ksize() within the kmalloc-backed mempools
except the mempool poisoning logic.  Use the actual pool size instead of
the ksize() to avoid needing any special handling of the memory as needed
by KASAN, UBSAN_BOUNDS, nor FORTIFY_SOURCE.

[vbabka@suse.cz: for slab mempools pool_data is not object size]
  Link: https://lkml.kernel.org/r/13c4bd6e-09d3-efce-43a5-5a99be8bc96b@suse.cz
Link: https://lkml.kernel.org/r/20221028154823.you.615-kees@kernel.org


Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Suggested-by: Vlastimil Babka <vbabka@suse.cz>
  Link: https://lore.kernel.org/lkml/f4fc52c4-7c18-1d76-0c7a-4058ea2486b9@suse.cz/


Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reported-by: Anders Roxell <anders.roxell@linaro.org>
  Link: https://lore.kernel.org/all/20221031105514.GB69385@mutt/


Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>