data in file did not match, during recursive add with local nocopy and fscache

#### Version information:

```
$ ipfs version --all
go-ipfs version: 0.4.13-
Repo version: 6
System version: amd64/linux
Golang version: go1.9.2
$ uname -a
Linux ipfsapt 4.13.0-26-generic #29~16.04.2-Ubuntu SMP Tue Jan 9 22:00:44 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
```
#### Type:

High - The main functionality of the application does not work, API breakage, repo format breakage, etc.

#### Description:

We are trying to add a full ubuntu mirror to IPFS.
Today, while running the command:
```
$ ipfs add --local=true --nocopy --fscache --recursive /data/repos/ubuntu/mirror/
```
If failed with:

```
11:20:54.548 ERROR commands/h: Failed to get block for zb2rhZzDivybfWjzSiMTq61gVCMqn2s1YokCDUV2nbUbBA6To: data in file did not match. repos/ubuntu/mirror/dists/artful-backports/universe/dep11/Components-amd64.yml.xz offset 0 client.go:247
Error: Failed to get block for zb2rhZzDivybfWjzSiMTq61gVCMqn2s1YokCDUV2nbUbBA6To: data in file did not match. repos/ubuntu/mirror/dists/artful-backports/universe/dep11/Components-amd64.yml.xz offset 0
```

I tried to add the file again, and it worked:

```
$ ipfs add --local=true --nocopy --fscache --recursive /data/repos/ubuntu/mirror/dists/artful-backports/universe/dep11/Components-amd64.yml.xz
added zb2rhoLrX6VKRtM5sk2tPXJ9Pd73mHqgTNgmm3SPHvFkL9xQx Components-amd64.yml.xz
```

This is pretty sad, because we can't resume the add. So for every error, we have to start again from scratch and wait a couple of hours at least.