erofs: enable iomap for file-backed mounts on unencoded inodes

Iomap supports non-bdev I/O through fs-provided callbacks since
commit b2f35ac4146d ("iomap: add caller-provided callbacks for read
and readahead"), making it suitable for file-backed mounts on unencoded
inodes.

Enable iomap for this case to support sub-folio buffered I/Os on
file-backed mounts and simplify the codebase.

Thanks-to: Hongbo Li <hongbohbli@tencent.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang (Docker) <xiang@kernel.org>