openvswitch: Add timeout support to ct action

Add support for fine-grain timeout support to conntrack action.
The new OVS_CT_ATTR_TIMEOUT attribute of the conntrack action
specifies a timeout to be associated with this connection.
If no timeout is specified, it acts as is, that is the default
timeout for the connection will be automatically applied.

Example usage:
$ nfct timeout add timeout_1 inet tcp syn_sent 100 established 200
$ ovs-ofctl add-flow br0 in_port=1,ip,tcp,action=ct(commit,timeout=timeout_1)

CC: Pravin Shelar <pshelar@ovn.org>
CC: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>