Power impact of debufferbloating

Solarflare Ethernet controllers support per-queue TX pacing. This was meant primarily as a means to throttle specific processes that are given their own queues, though so far I don't think we've made any use of it. You probably don't put 10G network controllers in a real low-power system though!

In the absence of pacing, it might be more useful to vary TX interrupt moderation (if there are separate interrupts for TX and RX).