kasan: improve free meta storage in Generic KASAN

kasan: improve free meta storage in Generic KASAN

Currently free meta can only be stored in object if the object is not
smaller than free meta.

After the improvement, even when the object is smaller than free meta, it
is still possible to store part of the free meta in the object, reducing
the increased size of the redzone.

Example:

free meta size: 16 bytes
alloc meta size: 16 bytes
object size: 8 bytes
optimal redzone size (object_size <= 64): 16 bytes

Before improvement:
actual redzone size = alloc meta size + free meta size = 32 bytes

After improvement:
actual redzone size = alloc meta size + (free meta size - object size)
                    = 24 bytes

Link: https://lkml.kernel.org/r/VI1P193MB0752DE2CCD9046B5FED0AA8E99B5A@VI1P193MB0752.EURP193.PROD.OUTLOOK.COM
Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Suggested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>