proctrack/cgroup: Move slurmstepd out of freezer cgroup before removal

In order to successfully remove the freezer cgroup at the end of
a job step, the slurmstepd process itself must first be moved
outside of the cgroup, or removal will always fail.

This fix moves the slurmstepd back to the root cgroup just
before the rmdir operations are attempted.

src/plugins/proctrack/cgroup/proctrack_cgroup.c

@@ -270,10 +270,27 @@ bail:
 	return SLURM_ERROR;
 }
 
+static int _move_current_to_root_cgroup(xcgroup_ns_t *ns)
+{
+	xcgroup_t cg;
+
+	if (xcgroup_create(ns, &cg, "", 0, 0) != XCGROUP_SUCCESS)
+		return SLURM_ERROR;
+
+	return xcgroup_move_process(&cg, getpid());
+}
+
 int _slurm_cgroup_destroy(void)
 {
 	xcgroup_lock(&freezer_cg);
 
+	/*
+	 *  First move slurmstepd process to the root cgroup, otherwise
+	 *   the rmdir(2) triggered by the calls below will always fail,
+	 *   because slurmstepd is still in the cgroup!
+	 */
+	_move_current_to_root_cgroup(&freezer_ns);
+
 	if (jobstep_cgroup_path[0] != '\0') {
 		if (xcgroup_delete(&step_freezer_cg) != XCGROUP_SUCCESS) {
 			debug("_slurm_cgroup_destroy: problem deleting step cgroup path %s: %m",