libct/cg/systemd/v1: fix err check in enableKmem

Commit 27d3dd3df3 ("don't fail when subsystem not mounted") added
ignoring "not found" error to enableKmem, and as a result the function
now tries to call Mkdir with an empty path, which results in a weird
error message. For example, this is a failure from a
libcontainer/integration test:

> === RUN   TestRunWithKernelMemorySystemd
>    exec_test.go:704: runContainer failed with kernel memory limit: container_linux.go:370: starting container process caused: process_linux.go:327: applying cgroup configuration for process caused: mkdir : no such file or directory

I am not entirely sure if it is a good idea to silently ignore set
limits, but at least let's fix the error handling.

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

libcontainer/cgroups/systemd/v1.go

@@ -406,7 +406,10 @@ func (m *legacyManager) Set(container *configs.Config) error {
 
 func enableKmem(c *configs.Cgroup) error {
 	path, err := getSubsystemPath(c, "memory")
-	if err != nil && !cgroups.IsNotFound(err) {
+	if err != nil {
+		if cgroups.IsNotFound(err) {
+			return nil
+		}
 		return err
 	}