Dont fail on missing subsystem for systemd stats

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)

cgroups/fs/apply_raw.go

@@ -75,6 +75,7 @@ func GetStats(c *cgroups.Cgroup) (*cgroups.Stats, error) {
 			if err == cgroups.ErrNotFound {
 				continue
 			}
+
 			return nil, err
 		}
 

cgroups/systemd/apply_systemd.go

@@ -422,6 +422,11 @@ func GetStats(c *cgroups.Cgroup) (*cgroups.Stats, error) {
 	for sysname, sys := range subsystems {
 		subsystemPath, err := getSubsystemPath(c, sysname)
 		if err != nil {
+			// Don't fail if a cgroup hierarchy was not found, just skip this subsystem
+			if err == cgroups.ErrNotFound {
+				continue
+			}
+
 			return nil, err
 		}