cgroup2_set_limits: fix harmless error message

In legacy cgroups mode, we were running `mountinfo -q ""` which was
generating an error message. If we return immediately when
cgroup2_find_path returns an empty value, we avoid this message.

sh/rc-cgroup.sh

@@ -189,6 +189,7 @@ cgroup2_set_limits()
 {
 	local cgroup_path
 	cgroup_path="$(cgroup2_find_path)"
+	[ -z "${cgroup_path}" ] && return 0
 	mountinfo -q "${cgroup_path}"|| return 0
 	rc_cgroup_path="${cgroup_path}/${RC_SVCNAME}"
 	[ ! -d "${rc_cgroup_path}" ] && mkdir "${rc_cgroup_path}"