Fixes logic for calculating percentage

Fixes logic for calculating percentage for cgroups.Stats.CpuStats.CpuUsage.PercentUsage

Previous logic divided a uint64 by a uint64 losing precision before multiplying by clock ticks and cpus
This fix maintains precision before converting back to uint64

Docker-DCO-1.1-Signed-off-by: Nicholas Weaver <lynxbat@gmail.com> (github: lynxbat)
WDYT of doing the multiplication first to not have to switch to floating point?