ofproto-dpif: Avoid figuring out sFlow and IPFIX actions twice.

ofproto-dpif: Avoid figuring out sFlow and IPFIX actions twice.

Not only is it easier to re-use the actions we already have, this avoids
potential problems due to the state that add_sflow_action() and
add_ipfix_action() look at having possibly been changed by
do_xlate_actions().  Currently those functions appear to look only at
the flow's 'in_port', which currently can't change.  However, an upcoming
commit will make it possible for actions to change the flow's 'in_port',
and in addition, with this change, one doesn't have to wonder whether these
functions might look at other state that translation might change.

Signed-off-by: Ben Pfaff <blp@nicira.com>