Silence warning about BPF firewall in containers

Silence warning about BPF firewall in containers

We'd get a warning on every nspawn invocation:
dev-hugepages.mount: unit configures an IP firewall, but the local system does not support BPF/cgroup firewalling.
(This warning is only shown for the first unit using IP firewalling.)

Before the previous commit, I'd generally get a warning about systemd-udev.service, even though that service is
not started in containers. But are still many other units which that declare a
firewall, which is currently unsupported in containers. Let's stop warning
about this.

The warning is still emitted e.g. if legacy cgroups are used. This is something
that can be configured, so it makes more sense to emit the warning.