rafs: fix amplify can not be skipped.

[ccx1024cc]

Production: With {.amplify_io} = 0, minimal read size from storage still remains 128K.

{
    "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
    }
}

Reason

{.fs_prefetch.merging_size} is used, instead of {.amplify_io}

[anolis-bot]

@ccx1024cc , a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/56433

[anolis-bot]

@ccx1024cc , the title has been updated, so a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/56435

[anolis-bot]

@ccx1024cc , the code has been updated, so a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/56436

[imeoer]

cc @jiangliu

[anolis-bot]

@ccx1024cc , The CI test is completed, please check result:

Test Case		Test Result
build rust golang image		✅ SUCCESS
compile nydusd		✅ SUCCESS
compile ctr remote		✅ SUCCESS
compile nydus snapshotter		✅ SUCCESS
run container with rafs		✅ SUCCESS
run container with zran		✅ SUCCESS
run container with rafs and compile linux		✅ SUCCESS

Congratulations, your test job passed!

[anolis-bot]

@ccx1024cc , The CI test is completed, please check result:

Test Case		Test Result
build rust golang image		✅ SUCCESS
compile nydusd		✅ SUCCESS
compile ctr remote		✅ SUCCESS
compile nydus snapshotter		✅ SUCCESS
run container with rafs		✅ SUCCESS
run container with zran		✅ SUCCESS
run container with rafs and compile linux		✅ SUCCESS

Congratulations, your test job passed!

[anolis-bot]

@ccx1024cc , The CI test is completed, please check result:

Test Case		Test Result
build rust golang image		✅ SUCCESS
compile nydusd		✅ SUCCESS
compile ctr remote		✅ SUCCESS
compile nydus snapshotter		✅ SUCCESS
run container with rafs		✅ SUCCESS
run container with zran		✅ SUCCESS
run container with rafs and compile linux		✅ SUCCESS

Congratulations, your test job passed!