python: Access available_filter_functions via TRACEFS

[lorenzph]

The /sys/kernel/debug/tracing mountpoint is considered deprecated and
is only enabled when the CONFIG_TRACEFS_AUTOMOUNT_DEPRECATED kernel
option is set. Some distributions, such as ArchLinux, no longer set
this, which breaks the use of get_kprobe_functions:

Traceback (most recent call last):
  File "biosnoop.py", line 349, in <module>
    if BPF.get_kprobe_functions(b'blk_start_request'):
       ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/bcc/__init__.py", line 735, in get_kprobe_functions
    raise e
  File "/usr/lib/python3.13/site-packages/bcc/__init__.py", line 731, in get_kprobe_functions
    with open(avail_filter_file, "rb") as avail_filter_f:
         ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/sys/kernel/debug/tracing/available_filter_functions'

Fix this by using the TRACEFS constant (which is set to the correct
tracefs mountpoint) instead.