kasan: use IS_ALIGNED in memory_is_poisoned_8()

Use IS_ALIGNED() to determine whether the shadow span two bytes.  It
generates less code and more readable.  Also add some comments in shadow
check functions.

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Andrey Konovalov <adech.fo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>