userswap: support userswap via userfaultfd

hulk inclusion
category: feature
bugzilla: 47439
CVE: NA

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This patch modify the userfaultfd to support userswap. To check whether
tha pages are dirty since the last swap in, we make them clean when we
swap in the pages. The userspace may swap in a large area and part of it
are not swapped out. We need to skip those pages that are not swapped
out.

Signed-off-by: Guo Fan <guofan5@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Reviewed-by: Kefeng  Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>