MEMFD_OPEN path is broken in the probes

The path generated by MEMFD_OPEN is broken:
```
      quark-test-3193771 [005] ...11 1601168.377267: bpf_trace_printk: buf_filename=memfd:
      quark-test-3193771 [005] ...11 1601168.377277: bpf_trace_printk: path=.
```

While it correctly generates a MEMFD_OPEN event from the probes, it puts a bogus path, this likely never worked as it is not used by anyone.
