UPSTREAM: erofs: insert to managed cache after adding to pcl

Previously, it could be some concern to call add_to_page_cache_lru()
with page->mapping == Z_EROFS_MAPPING_STAGING (!= NULL).

In contrast, page->private is used instead now, so partially revert
commit 5ddcee1f ("erofs: get rid of __stagingpage_alloc helper")
with some adaption for simplicity.

Link: https://lore.kernel.org/r/20201208095834.3133565-2-hsiangkao@redhat.com


Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Huang Jianan <huangjianan@oppo.com>

Bug: 190585249
Change-Id: Ia67c77a85f1e22bce2b1c0f238639fbb615d613a
(cherry picked from commit bf225074)
Signed-off-by: Huang Jianan <huangjianan@oppo.com>