softirqs fails to run

When running softirqs, I run into the following exception.

This issue is seen on Debian, with Debian packaged bpfcc.

```
rs@learner:~$ sudo softirqs
write(/sys/kernel/debug/tracing/kprobe_events, "p:kprobes/p_blk_iopoll_softirq blk_iopoll_softirq") failed: No such file or directory
c_void_p(None)
Traceback (most recent call last):
File "/usr/sbin/softirqs", line 118, in
b.attach_kprobe(event=softirqfunc, fn_name="trace_start")
File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 419, in attach_kprobe
raise Exception("Failed to attach BPF to kprobe")
Exception: Failed to attach BPF to kprobe
2016-12-12 / 18:08:38 ♒♒♒ ☹ => 1
rrs@learner:~$ sudo ls -l /sys/kernel/debug/tracing/kprobe_events
-rw-r--r-- 1 root root 0 Dec 11 18:49 /sys/kernel/debug/tracing/kprobe_events
2016-12-12 / 18:09:04 ♒♒♒ ☺
```

Start by reproducing `sudo softirqs` on Debian with the Debian-packaged bpfcc and inspect the reported write to `/sys/kernel/debug/tracing/kprobe_events`. Compare the failing `blk_iopoll_softirq` kprobe setup with the `softirqs` entry point shown in the traceback. Done means the command attaches its probes and runs without the reported exception.

Written by the indexing model from the issue text.