restore: Make sure the last_pid is writen with zero offset

Otherwise I see on 3.16-rc1 and higher

| [  100.851730] futex wrote to ns_last_pid when file position was not 0!
| This will not be supported in the future. To silence this
| warning, set kernel.sysctl_writes_strict = -1

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>