kasan: test: avoid writing invalid memory

Multiple KASAN tests do writes past the allocated objects or writes to
freed memory.  Turn these writes into reads to avoid corrupting memory.
Otherwise, these tests might lead to crashes with the HW_TAGS mode, as it
neither uses quarantine nor redzones.

Link: https://lkml.kernel.org/r/c3cd2a383e757e27dd9131635fc7d09a48a49cf9.1628779805.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>