mm/userswap: add VM_USWAP and SWP_USERSWAP_ENTRY

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


CVE: NA

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VM_USWAP gets set in vma->vm_flags to tell the VM common code that the
userswap is registered. SWP_USERSWAP_ENTRY is the swap entry when
userswap memory is swapped out. In addition, is_userswap_entry() is
introduced to determine whether the entry is a userswap swap entry. Add
the userswap entry case in zap_pte_range() to prevent WARN_ON_ONCE(1).

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