seize: fix error handling for check_freezer_cgroup

When `check_freezer_cgroup()` has non-zero return value, `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 properly.

This problem is related to #2508

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

criu/seize.c

@@ -1026,6 +1026,7 @@ int collect_pstree(void)
 	if (ret < 0 && ret != -ENOTSUP) {
 		goto err;
 	}
+	ret = -1;
 
 	if (opts.freeze_cgroup && cgroup_version())
 		goto err;