userswap: add a kernel parameter to enable userswap

hulk inclusion
category: bugfix
bugzilla: 175146
CVE: NA

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Disable userswap by default and add a kernel parameter to enable it.

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>

 Conflicts:
	include/linux/userfaultfd_k.h
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>