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.comSigned-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>

io_uring/alloc_cache.h

@@ -33,7 +33,7 @@ static inline struct io_cache_entry *io_alloc_cache_get(struct io_alloc_cache *c
 		struct io_cache_entry *entry;
 
 		entry = container_of(cache->list.next, struct io_cache_entry, node);
-		kasan_unpoison_range(entry, cache->elem_size);
+		kasan_mempool_unpoison_object(entry, cache->elem_size);
 		cache->list.next = cache->list.next->next;
 		cache->nr_cached--;
 		return entry;