selftests/bpf: Clean up btf and btf_dump in dump_datasec test

Free up used resources at the end and on error. Also make it more
obvious that there is btf__parse() call that creates struct btf
instance.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Hengqi Chen <hengqi.chen@gmail.com>
Link: https://lore.kernel.org/bpf/20211107165521.9240-7-andrii@kernel.org