staging: erofs: revisit the page submission flow

staging: erofs: revisit the page submission flow

Previously, the submission flow works with cached compressed pages
reclaim path in a tricky way, and it could be buggy if the reclaim
path changes later without such tricky restrictions. For example,
currently one PagePrivate(page) is evaluated without taking page
lock (it only follows a wait_for_page_locked which closes such race)
and no handling solves the potential page truncation case.

In addition, it's also full of #ifdefs in the function, which
is hard to understand and maintain. this patch fixes them all.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>