erofs: enable large folios for fscache mode

erofs: enable large folios for fscache mode

Enable large folios for fscache mode.  Enable this feature for
non-compressed format for now, until the compression part supports large
folios later.

One thing worth noting is that, the feature is not enabled for the meta
data routine since meta inodes don't need large folios for now, nor do
they support readahead yet.

Also document this new feature.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Link: https://lore.kernel.org/r/20221201074256.16639-3-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>