erofs: convert z_erofs_do_read_page() to folios

It is a straight-forward conversion. Besides, it's renamed as
z_erofs_scan_folio().

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240305091448.1384242-2-hsiangkao@linux.alibaba.com