sched: act_ct: switch to per-action label counting

net->ct.labels_used was meant to convey 'number of ip/nftables rules
that need the label extension allocated'.

act_ct enables this for each net namespace, which voids all attempts
to avoid ct->ext allocation when possible.

Move this increment to the control plane to request label extension
space allocation only when its needed.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>