service: make move to sub-cgroup non fatal

if we are running in a container in the root cgroup, Podman tries to
move itself to a sub-cgroup.  This could be a problem in a setup where
the cgroups are not writeable, so just log a debug message and
continue, since anyway it is a best-effort operation.

Closes: https://github.com/containers/podman/issues/15498

[NO NEW TESTS NEEDED]

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

cmd/podman/system/service_abi.go

@@ -105,7 +105,9 @@ func restService(flags *pflag.FlagSet, cfg *entities.PodmanConfig, opts entities
 	}
 
 	if err := utils.MaybeMoveToSubCgroup(); err != nil {
-		return err
+		// it is a best effort operation, so just print the
+		// error for debugging purposes.
+		logrus.Debugf("Could not move to subcgroup: %v", err)
 	}
 
 	servicereaper.Start()