io_uring: use mempool KASAN hook

Use the proper kasan_mempool_unpoison_object hook for unpoisoning cached
objects.

A future change might also update io_uring to check the return value of
kasan_mempool_poison_object to prevent double-free and invalid-free bugs. 
This proves to be non-trivial with the current way io_uring caches
objects, so this is left out-of-scope of this series.

Link: https://lkml.kernel.org/r/eca18d6cbf676ed784f1a1f209c386808a8087c5.1703024586.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>