ovs: allow nl 'flow set' to use ufid without flow key

When we want to change a flow using netlink, we have to identify it to
be able to perform a lookup. Both the flow key and unique flow ID
(ufid) are valid identifiers, but we always have to specify the flow
key in the netlink message. When both attributes are there, the ufid
is used. The flow key is used to validate the actions provided by
the userland.

This commit allows to use the ufid without having to provide the flow
key, as it is already done in the netlink 'flow get' and 'flow del'
path. The flow key remains mandatory when an action is provided.

Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>