biosnoop,biolatency,biotop,xfsslower run failed at centos7.6.1810

kernel: 3.10.0-957.21.2.el7.x86_64
centos: CentOS Linux release 7.6.1810

the failed log of run `biosnoop/biolatency/biotop`:
```
Traceback (most recent call last):
File "./biosnoop", line 159, in
if BPF.get_kprobe_functions(b'blk_start_request'):
File "/usr/lib/python2.7/site-packages/bcc/__init__.py", line 528, in get_kprobe_functions
with open("%s/../kprobes/blacklist" % TRACEFS, "rb") as blacklist_f:
IOError: [Errno 2] No such file or directory: '/sys/kernel/debug/tracing/../kprobes/blacklist'
```

the failed log of run `xfsslower`:
```
/virtual/main.c:50:18: error: incomplete definition of type 'struct kiocb'
val.fp = iocb->ki_filp;
~~~~^
/lib/modules/3.10.0-957.21.2.el7.x86_64/build/include/linux/fs.h:43:8: note: forward declaration of 'struct kiocb'
struct kiocb;
^
/virtual/main.c:51:22: error: incomplete definition of type 'struct kiocb'
val.offset = iocb->ki_pos;
~~~~^
/lib/modules/3.10.0-957.21.2.el7.x86_64/build/include/linux/fs.h:43:8: note: forward declaration of 'struct kiocb'
struct kiocb;
^
2 errors generated.
Traceback (most recent call last):
File "./xfsslower", line 264, in
b = BPF(text=bpf_text)
File "/usr/lib/python2.7/site-packages/bcc/__init__.py", line 325, in __init__
raise Exception("Failed to compile BPF text")
Exception: Failed to compile BPF text
```

Start by reproducing biosnoop, biolatency, biotop, and xfsslower on CentOS 7.6.1810 with kernel 3.10.0-957.21.2.el7.x86_64, using the reported traceback and compiler errors as entry points. Inspect the affected tool code and BCC behavior; done means these tools run successfully on the reported environment.

Written by the indexing model from the issue text.