mm/ksm: use helper macro __ATTR_RW

Use helper macro __ATTR_RW to define KSM_ATTR to make code more clear.
Minor readability improvement.

Link: https://lkml.kernel.org/r/20220221115809.26381-1-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>