UPSTREAM: erofs: fix up inplace I/O pointer for big pcluster

When picking up inplace I/O pages, it should be traversed in reverse
order in aligned with the traversal order of file-backed online pages.
Also, index should be updated together when preloading compressed pages.

Previously, only page-sized pclustersize was supported so no problem
at all. Also rename `compressedpages' to `icpage_ptr' to reflect its
functionality.

Link: https://lore.kernel.org/r/20210407043927.10623-5-xiang@kernel.org


Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

Bug: 201372112
Change-Id: I752769d004c18f74636bcfdea767572daa6f7072
(cherry picked from commit 81382f5f5cb0c9c5694c19d36460f757a8c96841)
Signed-off-by: Huang Jianan <huangjianan@oppo.com>