Don't fail on setting -ve oom score when rootless

This allows non-privileged users to use containerd.
If a non root user tried to set a negative oom score adjustment,
it will fail. Containerd should not fail if running rootless.

This is part of a larger track of work integrating containerd
into Cloudfoundry's garden with support for rootless.

[#156343443]

Signed-off-by: Danail Branekov <danailster@gmail.com>

sys/oom_unix.go

@@ -38,7 +38,7 @@ func SetOOMScore(pid, score int) error {
 	}
 	defer f.Close()
 	if _, err = f.WriteString(strconv.Itoa(score)); err != nil {
-		if os.IsPermission(err) && system.RunningInUserNS() {
+		if os.IsPermission(err) && (system.RunningInUserNS() || RunningUnprivileged()) {
 			return nil
 		}
 		return err