cgroup: fix regression when running systemd

commit 223fe64dc0 introduced the
regression.

When running on cgroups v1, bind mount only /sys/fs/cgroup/systemd as
rw, as the code did earlier.

Also, simplify the rootless code as it doesn't require any special
handling when using --systemd.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1737554

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

libpod/container_internal_linux.go

@@ -471,9 +471,9 @@ func (c *Container) setupSystemd(mounts []spec.Mount, g generate.Generator) erro
 		return err
 	}
 
+	if unified {
 		g.RemoveMount("/sys/fs/cgroup")
 
-	if unified {
 		sourcePath := filepath.Join("/sys/fs/cgroup")
 		systemdMnt := spec.Mount{
 			Destination: "/sys/fs/cgroup",
@@ -483,32 +483,14 @@ func (c *Container) setupSystemd(mounts []spec.Mount, g generate.Generator) erro
 		}
 		g.AddMount(systemdMnt)
 	} else {
-		// rootless containers have no write access to /sys/fs/cgroup, so don't
-		// add any mount into the container.
-		if !rootless.IsRootless() {
-			cgroupPath, err := c.CGroupPath()
-			if err != nil {
-				return err
-			}
-			sourcePath := filepath.Join("/sys/fs/cgroup", cgroupPath)
-
 		systemdMnt := spec.Mount{
-				Destination: "/sys/fs/cgroup",
+			Destination: "/sys/fs/cgroup/systemd",
 			Type:        "bind",
-				Source:      sourcePath,
+			Source:      "/sys/fs/cgroup/systemd",
-				Options:     []string{"bind", "private"},
-			}
-			g.AddMount(systemdMnt)
-		} else {
-			systemdMnt := spec.Mount{
-				Destination: "/sys/fs/cgroup",
-				Type:        "bind",
-				Source:      "/sys/fs/cgroup",
 			Options:     []string{"bind", "nodev", "noexec", "nosuid"},
 		}
 		g.AddMount(systemdMnt)
 	}
-	}
 
 	return nil
 }