kasan: clarify comment in __kasan_kfree_large

Currently it says that the memory gets poisoned by page_alloc code.
Clarify this by mentioning the specific callback that poisons the
memory.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://linux-review.googlesource.com/id/I1334dffb69b87d7986fab88a1a039cc3ea764725