openvswitch: Serialize nested ct actions if provided

If userspace provides a ct action with no nested mark or label, then the
storage for these fields is zeroed. Later when actions are requested,
such zeroed fields are serialized even though userspace didn't
originally specify them. Fix the behaviour by ensuring that no action is
serialized in this case, and reject actions where userspace attempts to
set these fields with mask=0. This should make netlink marshalling
consistent across deserialization/reserialization.

Reported-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>