biotop crashes on kernel 5.8.0-rc5: cannot attach kprobe, probe entry may not exist

```
# /usr/share/bcc/tools/biotop
cannot attach kprobe, probe entry may not exist
Traceback (most recent call last):
File "/usr/share/bcc/tools/biotop", line 181, in
b.attach_kprobe(event="blk_account_io_completion",
File "/usr/lib/python3.8/site-packages/bcc/__init__.py", line 676, in attach_kprobe
raise Exception("Failed to attach BPF program %s to kprobe %s" %
Exception: Failed to attach BPF program b'trace_req_completion' to kprobe b'blk_account_io_completion'
```
kernel-5.8.0-0.rc5.1.fc33.x86_64+debug
clang-10.0.0-2.fc32.x86_64
bcc-tools-0.15.0-2.fc32.x86_64

Works OK with kernel-5.7.6-200.fc32.x86_64

Start with /usr/share/bcc/tools/biotop and the attach_kprobe call for blk_account_io_completion, then inspect the bcc/__init__.py attach path shown in the traceback. Reproduce on kernel 5.8.0-rc5 and compare with 5.7.6; done means biotop attaches successfully and runs without the reported crash.

Written by the indexing model from the issue text.