mm/userswap: add enable_userswap boot option

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8KESX


CVE: NA

--------------------------------

Add enable_userswap boot option to enable userswap feature. Add static
key userswap_enabled to indicate whether the feature is enabled.

Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>