Dont ignore failure to create cgroup after timeout

Before this commit, creating a cgroup would silently ignore timeouts and
carry on. Concretely, this caused cases where a cgroup failed to create,
but the caller doesn't realize and ends up looking for files that should
exist (e.g. cgroups.controllers), only to find they don't exist. It's
very difficult as a caller to deal with this case, where NewSystemd
succeeds but the group doesn't exist.

The origins of this code seem to trace back to an initial implementation
written 5+ years ago:
5efa14e#diff-3331981e4ac06a8d9b06e91842b7f2759c7af3b65287e489a88385948d311ebdR672

runc added roughly the same logic here to deal with the same issue:
opencontainers/runc#3782

Now, containerd will also error if a cgroup cannot be created within the
timeout window.

Signed-off-by: Josh Chorlton <jchorlton@gmail.com>

cgroup2/manager.go

@@ -952,14 +952,16 @@ func startUnit(conn *systemdDbus.Conn, group string, properties []systemdDbus.Pr
 		}
 	}
 
+	systemdStartUnitTimeout := 30 * time.Second
 	select {
 	case s := <-statusChan:
 		if s != "done" {
 			attemptFailedUnitReset(conn, group)
 			return fmt.Errorf("error creating systemd unit `%s`: got `%s`", group, s)
 		}
-	case <-time.After(30 * time.Second):
-		log.G(ctx).Warnf("Timed out while waiting for StartTransientUnit(%s) completion signal from dbus. Continuing...", group)
+	case <-time.After(systemdStartUnitTimeout):
+		attemptFailedUnitReset(conn, group)
+		return fmt.Errorf("Timed out while waiting for StartTransientUnit(%s) completion signal from dbus after %v", group, systemdStartUnitTimeout)
 	}
 
 	return nil