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, when the object is smaller than free meta and SLUB
DEBUG is not enabled, it is possible to store part of the free meta in the
object, reducing the increased size of the red zone.

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

[juntong.deng@outlook.com: make kasan_metadata_size() adapt to the improved free meta storage]
  Link: https://lkml.kernel.org/r/VI1P193MB0752675D6E0A2D16CE656F8299BAA@VI1P193MB0752.EURP193.PROD.OUTLOOK.COM
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>