dpif-linux: Always pass an actions attribute in dpif_flow_put().

dpif-linux: Always pass an actions attribute in dpif_flow_put().

The kernel expects that ODP_FLOW_NEW always has an ODP_FLOW_ATTR_ACTIONS
attribute, even though that attribute may be empty to drop all of the
packets in the flow.  Similarly, ODP_FLOW_SET as used by
dpif_linux_flow_put() should always have such an attribute, since it is
used by OVS to update the flow's actions.  So make it possible for
dpif_linux_flow_to_ofpbuf() to pass an empty actions attribute, and make
dpif_linux_flow_put() always force that behavior if the actions_len passed
to it is 0.

This fixes EINVAL error creating flows to drop packets.

Acked-by: Jesse Gross <jesse@nicira.com>