net/sched: act_ct: fix ref leak when switching zones

In the Linux kernel, the following vulnerability has been resolved:

net/sched: act_ct: fix ref leak when switching zones

When switching zones or network namespaces without doing a ct clear in
between, it is now leaking a reference to the old ct entry. That's
because tcf_ct_skb_nfct_cached() returns false and
tcf_ct_flow_table_lookup() may simply overwrite it.

The fix is to, as the ct entry is not reusable, free it already at
tcf_ct_skb_nfct_cached().