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>

pie/restorer.c

@@ -887,6 +887,7 @@ long __export_restore_task(struct task_restore_args *args)
 
 			new_sp = restorer_stack(thread_args + i);
 			last_pid_len = vprint_num(last_pid_buf, sizeof(last_pid_buf), thread_args[i].pid - 1, &s);
+			sys_lseek(fd, 0, SEEK_SET);
 			ret = sys_write(fd, s, last_pid_len);
 			if (ret < 0) {
 				pr_err("Can't set last_pid %ld/%s\n", ret, last_pid_buf);