openvswitch: use nf_ct_get_tuplepr, invert_tuplepr

These versions deal with the l3proto/l4proto details internally.
It removes only caller of nf_ct_get_tuple, so make it static.

After this, l3proto->get_l4proto() can be removed in a followup patch.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>