erofs: use memcpy_to_folio() to replace copy_to_iter()

Using copy_to_iter() here is overkill and even messy.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250709034614.2780117-1-hsiangkao@linux.alibaba.com