rafs: fix amplify can not be skipped

``` json
{
    "device":{
        "backend":{
            "type":"registry",
            "config":{
                "readahead":false,
                "host":"dockerhub.kubekey.local",
                "repo":"dfns/alpine",
                "auth":"YWRtaw46SGFyYm9VMTIZNDU=",
                "scheme":"https",
                "skip_verify":true,
                "proxy":{
                    "fallback":false
                }
            }
        },
        "cache":{
            "type":"",
            "config":{
                "work_dir":"/var/lib/containerd-nydus/cache",
                "disable_indexed_map":false
            }
        }
    },
    "mode":"direct",
    "digest_validate":false,
    "jostats_files":true,
    "enable_xattr":true,
    "access_pattern":true,
    "latest_read_files":true,
    "batch_size":0,
    "amplify_io":0,
    "fs_prefetch":{
        "enable":false,
        "prefetch_all":false,
        "threads_count":10,
        "merging_size":131072,
        "bandwidth_rate":1048576,
        "batch_size":0,
        "amplify_io":0
    }
}
```
`{.fs_prefetch.merging_size}` is used, instead of `{.amplify_io}`

Signed-off-by: 泰友 <cuichengxu.ccx@antgroup.com>

rafs/src/fs.rs

@@ -102,7 +102,7 @@ impl Rafs {
             initialized: false,
             digest_validate: rafs_cfg.validate,
             fs_prefetch: rafs_cfg.prefetch.enable,
-            amplify_io: rafs_cfg.prefetch.batch_size as u32,
+            amplify_io: rafs_cfg.batch_size as u32,
             prefetch_all: rafs_cfg.prefetch.prefetch_all,
             xattr_enabled: rafs_cfg.enable_xattr,