seize: fix error handling for check_freezer_cgroup

When check_freezer_cgroup() has non-zero, `goto err` calls `return ret`.
However, the value of `ret` has been set to 0 in the lines above and
CRIU does not handle the error.

This problem is related to #2508

Signed-off-by: Radostin Stoyanov <rstoyanov@fedoraproject.org>

criu/seize.c

@@ -1032,6 +1032,7 @@ int collect_pstree(void)
 
 	pr_debug("Detected cgroup V%d freezer\n", cgroup_v2 ? 2 : 1);
 
+	ret = -1;
 	if (opts.freeze_cgroup && !freeze_cgroup_disabled) {
 		if (freeze_processes())
 			goto err;