kasan: test: avoid corrupting memory in copy_user_test

kasan: test: avoid corrupting memory in copy_user_test

copy_user_test() does writes past the allocated object.  As the result, it
corrupts kernel memory, which might lead to crashes with the HW_TAGS mode,
as it neither uses quarantine nor redzones.

(Technically, this test can't yet be enabled with the HW_TAGS mode, but
this will be implemented in the future.)

Adjust the test to only write memory within the aligned kmalloc object.

Link: https://lkml.kernel.org/r/19bf3a5112ee65b7db88dc731643b657b816c5e8.1628779805.git.andreyknvl@gmail.com
Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>