libct/cg/sd/v2: unifiedResToSystemdProps nit

In code that checks that the resource name is in the for
Using strings.SplitN is an overkill in this case, resulting in
allocations and thus garbage to collect.

Using strings.IndexByte and checking that result is not less than 1
(meaning there is a period, and it is not the first character) is
sufficient here.

Fixes: 0cb8bf6
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/v2.go

@@ -64,8 +64,7 @@ func unifiedResToSystemdProps(cm *dbusConnManager, res map[string]string) (props
 		if strings.Contains(k, "/") {
 			return nil, fmt.Errorf("unified resource %q must be a file name (no slashes)", k)
 		}
-		sk := strings.SplitN(k, ".", 2)
-		if len(sk) != 2 {
+		if strings.IndexByte(k, '.') < 2 {
 			return nil, fmt.Errorf("unified resource %q must be in the form CONTROLLER.PARAMETER", k)
 		}
 		// Kernel is quite forgiving to extra whitespace