kasan: update kasan_poison documentation comment

kasan: update kasan_poison documentation comment

The comment for kasan_poison says that the size argument gets aligned by
the function to KASAN_GRANULE_SIZE, which is wrong: the argument must be
already aligned when it is passed to the function.

Remove the invalid part of the comment.

Link: https://lkml.kernel.org/r/992a302542059fc40d86ea560eac413ecb31b6a1.1703188911.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>