scripts: documentation-file-ref-check: fix bpf selftests path

tools/testing/selftests/bpf/test_bpftool_synctypes.py use
relative patches on the top of BPFTOOL_DIR:

	BPFTOOL_DIR = os.path.join(LINUX_ROOT, 'tools/bpf/bpftool')

Change the script to automatically convert:

	testing/selftests/bpf -> bpf/bpftool

In order to properly check the files used by such script.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/49b765cbac6ccd22d627573154806ec9389d60f0.1634629094.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>