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.

Link: https://lkml.kernel.org/r/1c8380fe0332a3bcc720fe29f1e0bef2e2974416.1606162397.git.andreyknvl@google.com
Link: https://linux-review.googlesource.com/id/I1334dffb69b87d7986fab88a1a039cc3ea764725Signed-off-by: NAndrey Konovalov <andreyknvl@google.com>
Reviewed-by: NDmitry Vyukov <dvyukov@google.com>
Reviewed-by: NMarco Elver <elver@google.com>
Tested-by: NVincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Branislav Rankov <Branislav.Rankov@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/kasan/common.c

@@ -446,5 +446,5 @@ void __kasan_kfree_large(void *ptr, unsigned long ip)
 {
 	if (ptr != page_address(virt_to_head_page(ptr)))
 		kasan_report_invalid_free(ptr, ip);
-	/* The object will be poisoned by page_alloc. */
+	/* The object will be poisoned by kasan_free_pages(). */
 }