outer helper: burn limits to the subcgroup, not the parent

[vzxv]

The outer helper currently burns limits into the wrong place, as evidenced by this strace:

open("/proc/self/cgroup", O_RDONLY|O_LARGEFILE) = 4
mkdirat(4, "bst.2482939", 0777) = 0
openat(4, "bst.2482939", O_RDONLY|O_LARGEFILE|O_DIRECTORY) = 6
openat(4, "memory.oom.group", O_WRONLY|O_LARGEFILE) = 7
write(7, "1", 1) = 1

This change fixes it.