erofs: switch to prepare_ondemand_read() in fscache mode

Switch to prepare_ondemand_read() interface and a self-contained request
completion to get rid of netfs_io_[request|subrequest].

The whole request will still be split into slices (subrequest) according
to the cache state of the backing file.  As long as one of the
subrequests fails, the whole request will be marked as failed.
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Link: https://lore.kernel.org/r/20221124034212.81892-3-jefflexu@linux.alibaba.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>