kasan: improve kasan_non_canonical_hook

kasan: improve kasan_non_canonical_hook

Make kasan_non_canonical_hook to be more sure in its report (i.e.  say
"probably" instead of "maybe") if the address belongs to the shadow memory
region for kernel addresses.

Also use the kasan_shadow_to_mem helper to calculate the original address.

Also improve the comments in kasan_non_canonical_hook.

Link: https://lkml.kernel.org/r/af94ef3cb26f8c065048b3158d9f20f6102bfaaa.1703188911.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>