cgroups/systemd: allow to set -1 as pids.limit

Currently, both systemd cgroup drivers (v1 and v2) only set
"TasksMax" unit property if the value > 0, so there is no
way to update the limit to -1 / unlimited / infinity / max.

Since systemd driver is backed by fs driver, and both fs and fs2
set the limit of -1 properly, it works, but systemd still has
the old value:

 # runc --systemd-cgroup update $CT --pids-limit 42
 # systemctl show runc-$CT.scope | grep TasksMax
 TasksMax=42
 # cat /sys/fs/cgroup/system.slice/runc-$CT.scope/pids.max
 42

 # ./runc --systemd-cgroup update $CT --pids-limit -1
 # systemctl show runc-$CT.scope | grep TasksMax=
 TasksMax=42
 # cat /sys/fs/cgroup/system.slice/runc-xx77.scope/pids.max
 max

Fix by changing the condition to allow -1 as a valid value.

NOTE other negative values are still being ignored by systemd drivers
(as it was done before). I am not sure whether this is correct, or
should we return an error.

A test case is added.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/v1.go

@@ -125,7 +125,7 @@ func genV1ResourcesProperties(c *configs.Cgroup) ([]systemdDbus.Property, error)
 			newProp("BlockIOWeight", uint64(c.Resources.BlkioWeight)))
 	}
 
-	if c.Resources.PidsLimit > 0 {
+	if c.Resources.PidsLimit > 0 || c.Resources.PidsLimit == -1 {
 		properties = append(properties,
 			newProp("TasksAccounting", true),
 			newProp("TasksMax", uint64(c.Resources.PidsLimit)))