cgroup: make memory.kmem.limit_in_bytes a "special" property

This property can't be written after there are children of the current
cgroup or once it has tasks, similar to the cpuset properties. So, let's
hoist this property into the "special" array.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>