daemon: don't adjust oom-score if score is 0

This patch makes two changes if --oom-score-adj is set to 0

- do not adjust the oom-score-adjust cgroup for dockerd
- do not set the hard-coded -999 score for containerd if
  containerd is running as child process

Before this change:

oom-score-adj | dockerd       | containerd as child-process
--------------|---------------|----------------------------
-             | -500          | -500 (same as dockerd)
-100          | -100          | -100 (same as dockerd)
 0            |  0            | -999 (hard-coded default)

With this change:

oom-score-adj | dockerd       | containerd as child-process
--------------|---------------|----------------------------
-             | -500          | -500 (same as dockerd)
-100          | -100          | -100 (same as dockerd)
0             | not adjusted  | not adjusted

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/daemon_unix.go

@@ -1636,6 +1636,9 @@ func setMayDetachMounts() error {
 }
 
 func setupOOMScoreAdj(score int) error {
+	if score == 0 {
+		return nil
+	}
 	f, err := os.OpenFile("/proc/self/oom_score_adj", os.O_WRONLY, 0)
 	if err != nil {
 		return err

libcontainerd/supervisor/remote_daemon_linux.go

@@ -28,9 +28,6 @@ func (r *remote) setDefaults() {
 	if r.Debug.Address == "" {
 		r.Debug.Address = filepath.Join(r.stateDir, debugSockFile)
 	}
-	if r.OOMScore == 0 {
-		r.OOMScore = -999
-	}
 
 	for key, conf := range r.pluginConfs.Plugins {
 		if conf == nil {