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.

src/python/bcc/__init__.py

@@ -546,8 +546,15 @@ def attach_raw_socket(fn, dev):
 
  @staticmethod
  def get_kprobe_functions(event_re):
- with open("%s/../kprobes/blacklist" % TRACEFS, "rb") as blacklist_f:
- blacklist = set([line.rstrip().split()[1] for line in blacklist_f])
+ blacklist_file = "%s/../kprobes/blacklist" % TRACEFS
+ try:
+ with open(blacklist_file, "rb") as blacklist_f:
+ blacklist = set([line.rstrip().split()[1] for line in blacklist_f])
+ except IOError as e:
+ if e.errno != errno.EPERM:
+ raise e
+ blacklist = set([])
+
  fns = []
 
  in_init_section = 0
@@ -607,7 +614,7 @@ def _del_kprobe_fd(self, name):
  global _num_open_probes
  del self.kprobe_fds[name]
  _num_open_probes -= 1
- 
+
  def _add_uprobe_fd(self, name, fd):
  global _num_open_probes
  self.uprobe_fds[name] = fd
@@ -643,7 +650,7 @@ def fix_syscall_fnname(self, name):
  if name.startswith(prefix):
  return self.get_syscall_fnname(name[len(prefix):])
  return name
- 
+
  def attach_kprobe(self, event=b"", event_off=0, fn_name=b"", event_re=b""):
  event = _assert_is_bytes(event)
  fn_name = _assert_is_bytes(fn_name)