cgroup: systemctl: Don't display NULL if protection was set to max

cgroup: systemctl: Don't display NULL if protection was set to max

Inside format_bytes, we return NULL if the value is UINT64_MAX. This
makes some kind of sense where this has some other semantic meaning than
being a value, but in this case the value is both a.) not the default
(so we definitely want to display it), and b.) means "infinity" (or
"max" in cgroup terminology).

This patch adds a small wrapper around format_bytes that can be used for
these cases, to avoid the following situation:

    [root@tangsanjiao ~]# cat /sys/fs/cgroup/workload.slice/memory.low
    max
    [root@tangsanjiao ~]# systemctl show workload.slice -p MemoryLow
    MemoryLow=infinity
    [root@tangsanjiao ~]# systemctl status workload.slice | grep low:
	Memory: 14.9G (low: (null))

After the patch:

    [root@tangsanjiao ~]# systemctl status workload.slice | grep low:
	Memory: 15.1G (low: infinity)