zdtm: reset file position to the begginning of the file in netns-dev

writing to netdev configuration at non zero offset won't be supported:

```
[  301.431638] netns-dev wrote to accept_local when file position was
not 0! This will not be supported in the future. To silence this
warning, set kernel.sysctl_writes_strict = -1
```

v2: fix compilation

Sorry, please don't consider v1.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@odin.com>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>