oomd: fix race with path unavailability when killing cgroups

oomd: fix race with path unavailability when killing cgroups

There can be a situation where systemd-oomd would kill all of the processes
in a cgroup, pid1 would clean up that cgroup, and systemd-oomd would get
ENODEV trying to iterate the cgroup a final time to ensure it was empty.
systemd-oomd sees this as an error and immediately picks a new candidate even
though pressure may have recovered. To counter this, check and handle
path unavailability errnos specially.

Fixes: #22030