replace bpf_ktime_get_ns with faster bpf_jiffies64

[lmb]

The single call to bpf_ktime_get_ns uses about 30% of benchmark time on my machine. It would be a big perf win if we can replace it with something else.

There is a function bpf_jiffies64 which is probably cheaper, but less accurate. Open questions:

• What is the resolution we can get from jiffies64, and is that enough for our purposes?
• How do we convert jiffies64 to a timestamp?