In https://github.com/torvalds/linux/commit/5ca3726 (released in v4.7-rc1) the
content of the `cpuacct.usage_percpu` file in sysfs was changed to include both
online and offline cpus. This broke the arithmetic in the stats helpers used by
`docker stats`, since it was using the length of the PerCPUUsage array as a
proxy for the number of online CPUs.
Add current number of online CPUs to types.StatsJSON and use it in the
calculation.
Keep a fallback to `len(v.CPUStats.CPUUsage.PercpuUsage)` so this code
continues to work when talking to an older daemon. An old client talking to a
new daemon will ignore the new field and behave as before.
Fixes#28941.
Signed-off-by: Ian Campbell <ian.campbell@docker.com>
