dpif-linux: Only ask datapath to echo back results when they will be used.

dpif-linux: Only ask datapath to echo back results when they will be used.

A fair number of datapath flow operations optionally report back results
to the requester based on whether NLM_F_ECHO is set in the request.  When
userspace isn't going to use those results anyway, it wastes memory to
store them and a system call to retrieve them.

This commit omits the NLM_F_ECHO bit in cases where the caller isn't going
to use the results.

(NLM_F_ECHO has no effect on operations whose entire purpose is to retrieve
data, e.g. "get" and "dump" operations, so we need not bother to set it
for those.)

This improves "ovs-benchmark rate" results in my testing by about 4%.