selftests/bpf: Variable naming fix

Change log_fd to log_fp to reflect its type correctly.

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211112192535.898352-3-fallentree@fb.com