ipfix: Bug fix for configuring IPFIX for flows

ipfix: Bug fix for configuring IPFIX for flows

There are two kinds of IPFIX: bridge level IPFIX and flow level
IPFIX. Now if we only configure flow level IPFIX, even if there
is no bridge IPFIX configuration, the datapath flow will contain
a sample action for bridge IPFIX. Fix it.

Steps to configure flow level IPFIX:
1) Create a new record in Flow_Sample_Collector_Set table:
   'ovs-vsctl -- create Flow_Sample_Collector_Set id=1 bridge="Bridge UUID"'
2) Add IPFIX configuration which is referred by corresponding
   row in Flow_Sample_Collector_Set table:
   'ovs-vsctl -- set Flow_Sample_Collector_Set
   "Flow_Sample_Collector_Set UUID" ipfix=@i -- --id=@i create IPFIX
   targets=\"IP:4739\" obs_domain_id=123 obs_point_id=456
   cache_active_timeout=60 cache_max_flows=13'
3) Add sample action to the flows:
   'ovs-ofctl add-flow mybridge in_port=1,
   actions=sample'('probability=65535,collector_set_id=1,
   obs_domain_id=123,obs_point_id=456')',output:LOCAL'

Before this fix, if you only configure flow IPFIX, the datapath flow is:
   id(0),in_port(2),eth_type(0x0806), packets:0, bytes:0, used:never,
   actions:sample(sample=0.0%,actions(userspace(pid=4294960835,
   ipfix(output_port=4294967295)))),sample(sample=100.0%,
   actions(userspace(pid=4294960835,flow_sample(probability=65535,
   collector_set_id=1,obs_domain_id=123,obs_point_id=456)))),
   sample(sample=0.0%,actions(userspace(pid=4294960835,
   ipfix(output_port=1)))),1

The datapath flow should only contain the sample action like below:
   id(0),in_port(2),eth_type(0x0800),ipv4(frag=no), packets:9, bytes:871,
   used:0.656s, actions:sample(sample=100.0%,actions(userspace(pid=4294962911,
   flow_sample(probability=65535,collector_set_id=1,obs_domain_id=123,
   obs_point_id=456)))),1

Signed-off-by: Benli Ye <daniely@vmware.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>