userswap: check read and write permissions for swap-out pages

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6CAIM



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Check the VM_READ and VM_WRITE flags of vma->vm_flags to determine
whether the read and write permissions of the swap-out page VA are
consistent with those of the swap-out buffer VA. If they are
inconsistent, the swap operation will fail.

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