userswap: fix BUG_ON in userfaultfd_release()

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



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In some features of userfaultfd, vma->vm_userfaultfd_ctx.ctx may be NULL
but VM_USWAP is not cleared. No longer check whether vma->vm_flags has
VM_USWAP. Just remove the flag.

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