selftests/bpf: Attach to fopen()/fclose() in attach_probe

malloc() and free() may be completely replaced by sanitizers, use
fopen() and fclose() instead.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230210001210.395194-7-iii@linux.ibm.com