erofs: convert z_erofs_onlinepage_.* to folios

Online folios are locked file-backed folios which will eventually
keep decoded (e.g. decompressed) data of each inode for end users to
utilize.  It may belong to a few pclusters and contain other data (e.g.
compressed data for inplace I/Os) temporarily in a time-sharing manner
to reduce memory footprints for low-ended storage devices with high
latencies under heary I/O pressure.

Apart from folio_end_read() usage, it's a straight-forward conversion.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240305091448.1384242-1-hsiangkao@linux.alibaba.com