this commit updates checkEphemeralStorage to be able to add container log stats, if applicable.
It also updates the old check when container log stats aren't found to be more accurate.
Specifically, this check previously worked because of a fluke programming accident:
according to this block in pkg/kubelet/stats/helper.go:113
```
if result.Rootfs != nil {
rootfsUsage := *cfs.BaseUsageBytes
result.Rootfs.UsedBytes = &rootfsUsage
}
```
BaseUsageBytes should be the value added, not TotalUsageBytes. However, since in this case
one also needs to account for the calculated log size, which is TotalUsageBytes - BaseUsageBytes
using TotalUsageBytes value accidentally worked.
Updating the case to use the correct value AND log offset fixes this accident and makes
the behavior more in line with what happens when calculating ephemeral storage.
Signed-off-by: Peter Hunt <pehunt@redhat.com>
