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 5ddcee1f3a1c ("erofs: get rid of __stagingpage_alloc helper")
with some adaption for simplicity.
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxx>