openvswitch: Update the CT state key only after nf_conntrack_in().

Only a successful nf_conntrack_in() call can effect a connection state
change, so it suffices to update the key only after the
nf_conntrack_in() returns.

This change is needed for the later NAT patches.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>