examples: Use pid as filter instead of tid and make some improvements for stacksnoop.py

[xingfeng2510]

For examples/stacksnoop.py:

• Use pid as filter instead of tid, keep consistent with -p option of other tools.
• Make some improvements (remove ctypes & 'b' prefixes).

[xingfeng2510]

ci failed due to following error (in mdflush.py, bio->bi_disk not found).

error: no member named 'bi_disk' in 'struct bio'
     struct gendisk *bi_disk = bio->bi_disk;
                              ~~~  ^
1 error generated.
Traceback (most recent call last):
   File "/bcc/tools/mdflush.py", line 54, in <module>

I guess CI runtime env (such as kernel version) had changed in recent days, and I will make another pull request to fix it.

[xingfeng2510]

CI build failure has been fixed in #3936.