[CRaC] Avoid error when running with -XX:+PerfDisableSharedMem

CRaC tries to unconditionally restore the hsperf file, even if the checkpointed JVM process was started with `-XX:+PerfDisableSharedMem`.

The fix is trivial: check for the existence of the hsperf file in `PerfMemoryLinux::restore()` just as we already do it in `PerfMemoryLinux::checkpoint()`.