erofs: convert z_erofs_pcluster_readmore() to folios

Unlike `pagecache_get_page()`, `__filemap_get_folio()` returns error
pointers instead of NULL, thus switching to `IS_ERR_OR_NULL`.

Apart from that, it's just a straightforward conversion.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240703120051.3653452-1-hsiangkao@linux.alibaba.com