Infinite loop when reading /proc/cpuinfo

When CPU limit is set to a higher value than the number of CPUs on the host system, reading
from `/proc/cpuinfo` in containers never ends, it keeps returning processor 0 over and over.

Reproducer on a system with 8 CPUs:

```
ct new --distribution alpine testct
ct set cpu-limit testct 1000
ct start testct
ct exec testct cat /proc/cpuinfo
```