selftests: openvswitch: Set value to nla flags.

Netlink flags, although they don't have payload at the netlink level,
are represented as having "True" as value in pyroute2.

Without it, trying to add a flow with a flag-type action (e.g: pop_vlan)
fails with the following traceback:

Traceback (most recent call last):
  File "[...]/ovs-dpctl.py", line 2498, in <module>
    sys.exit(main(sys.argv))
             ^^^^^^^^^^^^^^
  File "[...]/ovs-dpctl.py", line 2487, in main
    ovsflow.add_flow(rep["dpifindex"], flow)
  File "[...]/ovs-dpctl.py", line 2136, in add_flow
    reply = self.nlm_request(
            ^^^^^^^^^^^^^^^^^
  File "[...]/pyroute2/netlink/nlsocket.py", line 822, in nlm_request
    return tuple(self._genlm_request(*argv, **kwarg))
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[...]/pyroute2/netlink/generic/__init__.py", line 126, in
nlm_request
    return tuple(super().nlm_request(*argv, **kwarg))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[...]/pyroute2/netlink/nlsocket.py", line 1124, in nlm_request
    self.put(msg, msg_type, msg_flags, msg_seq=msg_seq)
  File "[...]/pyroute2/netlink/nlsocket.py", line 389, in put
    self.sendto_gate(msg, addr)
  File "[...]/pyroute2/netlink/nlsocket.py", line 1056, in sendto_gate
    msg.encode()
  File "[...]/pyroute2/netlink/__init__.py", line 1245, in encode
    offset = self.encode_nlas(offset)
             ^^^^^^^^^^^^^^^^^^^^^^^^
  File "[...]/pyroute2/netlink/__init__.py", line 1560, in encode_nlas
    nla_instance.setvalue(cell[1])
  File "[...]/pyroute2/netlink/__init__.py", line 1265, in setvalue
    nlv.setvalue(nla_tuple[1])
                 ~~~~~~~~~^^^
IndexError: list index out of range

Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>