erofs: convert z_erofs_bind_cache() to folios

The managed cache uses a pseudo inode to keep (necessary) compressed
data.

Currently, it still uses zero-order folios, so this is just a trivial
conversion, except that the use of the pagepool is temporarily dropped.

Drop some obsoleted comments too.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250114034429.431408-4-hsiangkao@linux.alibaba.com