selftests/bpf: Fix memory leaks in btf_type_c_dump() helper

Free up memory and resources used by temporary allocated memstream and
btf_dump 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/20211107165521.9240-4-andrii@kernel.org