dpctl: Fix broken flow deletion via ovs-dpctl due to missing ufid.

dpctl: Fix broken flow deletion via ovs-dpctl due to missing ufid.

Current code generates UFID for flows installed by ovs-dpctl.  This
leads to inability to remove such flows by the same command.  Ex:

  ovs-dpctl add-dp test
  ovs-dpctl add-if test vport0
  ovs-dpctl add-flow test "in_port(0),eth(),eth_type(0x800),ipv4(src=100.1.0.1)" 0
  ovs-dpctl del-flow test "in_port(0),eth(),eth_type(0x800),ipv4(src=100.1.0.1)"

  dpif|WARN|system@test: failed to flow_del (No such file or directory)
      ufid:e4457189-3990-4a01-bdcf-1e5f8b208711 in_port(0),
      eth(src=00:00:00:00:00:00,dst=00:00:00:00:00:00),eth_type(0x0800),
      ipv4(src=100.1.0.1,dst=0.0.0.0,proto=0,tos=0,ttl=0,frag=no)

  ovs-dpctl: deleting flow (No such file or directory)
  Perhaps you need to specify a UFID?

During del-flow operation UFID is generated too, however resulted
value is different from one generated during add-flow.  This happens
because odp_flow_key_hash() function uses random base value for flow
hashes which is different on every invocation.  That is not an issue
while running 'ovs-appctl dpctl/{add,del}-flow' because execution
of these requests happens in context of the OVS main process, i.e.
there will be same random seed.

Commit e61984e781e6 was intended to allow offloading for flows
added by dpctl/add-flow unixctl command, so it's better to generate
UFIDs conditionally inside dpctl command handler only for appctl
invocations.  Offloading is not possible from ovs-dpctl utility anyway.

There are still couple of corner case:  It will not be possible to
remove flow by 'ovs-appctl dpctl/del-flow' without specifying UFID if
main OVS process was restarted since flow addition and it will not
be possible to remove flow by ovs-dpctl without specifying UUID if
it was added by 'ovs-appctl dpctl/add-flow'.  But these scenarios
seems minor since these commands intended for testing only.

Reported-by: Eelco Chaudron <echaudro@redhat.com>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-dev/2020-September/374863.html
Fixes: e61984e781e6 ("dpif-netlink: Generate ufids for installing TC flowers")
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Tested-by: Eelco Chaudron <echaudro@redhat.com>