oomd: handle situations when no cgroups are killed

oomd: handle situations when no cgroups are killed

Currently if systemd-oomd doesn't kill anything in a selected cgroup, it
selects a new candidate immediately. But if a selected cgroup wasn't killed,
it is likely due to it disappearing or getting cleaned up between the time
it was selected as a candidate and getting sent SIGKILL(s). We should handle
it as though systemd-oomd did perform a kill so that it will check
swap/pressure again before it tries to select a new candidate.