cgroup: Fix clone_flags drop error in fork_with_pid

The commit dae14ddf does not drop the CLONE_NEWCGROUP correctly.
Thsi patch fix it.

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>