vma->vm_page_prot is read lockless from the rmap_walk, it may be updated
concurrently.  Using READ_ONCE to prevent the risk of reading intermediate
values.

Link: https://lkml.kernel.org/r/20220704132201.14611-3-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>