mm/userswap: support userswap via userfaultfd

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


CVE: NA

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This patch modify the userfaultfd to support userswap. VM_USWAP is set
in userfaultfd_register() and cleared in userfaultfd_unregister() and
userfaultfd_release(). Use do_uswap_page() to handle page faults of the
userswap swap entry in do_swap_page(). Add uswap_must_wait() to handle
userswap type userfaults in userfaultfd_must_wait().

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