Make reading blacklist from debugfs optional
Make reading blacklist from debugfs optional

With lockdown enabled one sees the following:

```
$ sudo /usr/share/bcc/tools/funccount -Ti 1 run_timer_softirq
[Errno 1] Operation not permitted: '/sys/kernel/debug/tracing/../kprobes/blacklist'
```

Which is accompanied by the following in `dmesg`:

```
[Fri May 29 22:12:47 2020] Lockdown: funccount: debugfs access is restricted; see man kernel_lockdown.7
```

Since blacklist is not a required feature, let's make
reading from it optional, so that bcc can work out of the box.

brendangregg/bcc,iovisor/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,tuxology/bcc,tuxology/bcc,iovisor/bcc,iovisor/bcc,tuxology/bcc,tuxology/bcc,brendangregg/bcc,brendangregg/bcc