erofs: convert z_erofs_fill_bio_vec() to folios

Introduce a folio member to `struct z_erofs_bvec` and convert most
of z_erofs_fill_bio_vec() to folios, which is still straight-forward.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240305091448.1384242-4-hsiangkao@linux.alibaba.com