Python: fix detach_kprobe()

As open_kprobes contains not fd but pointer to struct perf_reader,
it should call lib.perf_reader_free() instead of os.close()

src/python/bcc/__init__.py

@@ -575,7 +575,7 @@ def detach_kprobe(event):
  ev_name = "p_" + event.replace("+", "_").replace(".", "_")
  if ev_name not in open_kprobes:
  raise Exception("Kprobe %s is not attached" % event)
- os.close(open_kprobes[ev_name])
+ lib.perf_reader_free(open_kprobes[ev_name])
  desc = "-:kprobes/%s" % ev_name
  res = lib.bpf_detach_kprobe(desc.encode("ascii"))
  if res < 0:
@@ -612,7 +612,7 @@ def detach_kretprobe(event):
  ev_name = "r_" + event.replace("+", "_").replace(".", "_")
  if ev_name not in open_kprobes:
  raise Exception("Kretprobe %s is not attached" % event)
- os.close(open_kprobes[ev_name])
+ lib.perf_reader_free(open_kprobes[ev_name])
  desc = "-:kprobes/%s" % ev_name
  res = lib.bpf_detach_kprobe(desc.encode("ascii"))
  if res < 0: