kasan: introduce kasan_mempool_unpoison_object

Introduce and document a kasan_mempool_unpoison_object hook.

This hook serves as a replacement for the generic kasan_unpoison_range
that the mempool code relies on right now.  mempool will be updated to use
the new hook in one of the following patches.

For now, define the new hook to be identical to kasan_unpoison_range.  One
of the following patches will update it to add stack trace collection.

Link: https://lkml.kernel.org/r/dae25f0e18ed8fd50efe509c5b71a0592de5c18d.1703024586.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Lobakin <alobakin@pm.me>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>