cgroup: special handling of memory.oom_control

memory.oom_control when read displays something like:

root@kernel:/sys/fs/cgroup/memory/zdtmtst# cat memory.oom_control
oom_kill_disable 0
under_oom 0

but when setting it, it expects a "1" or "0" when written. Let's parse out
this 1 or 0.

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