staging: erofs: compressed_pages should not be accessed again after freed

commit af692e11 upstream.

This patch resolves the following page use-after-free issue,
z_erofs_vle_unzip:
    ...
    for (i = 0; i < nr_pages; ++i) {
        ...
        z_erofs_onlinepage_endio(page);  (1)
    }

    for (i = 0; i < clusterpages; ++i) {
        page = compressed_pages[i];

        if (page->mapping == mngda)      (2)
            continue;
        /* recycle all individual staging pages */
        (void)z_erofs_gather_if_stagingpage(page_pool, page); (3)
        WRITE_ONCE(compressed_pages[i], NULL);
    }
    ...

After (1) is executed, page is freed and could be then reused, if
compressed_pages is scanned after that, it could fall info (2) or
(3) by mistake and that could finally be in a mess.

This patch aims to solve the above issue only with little changes
as much as possible in order to make the fix backport easier.

Fixes: 3883a79a ("staging: erofs: introduce VLE decompression support")
Cc: <stable@vger.kernel.org> # 4.19+
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>