selftests/bpf: Add missing bpf_iter_vma_offset__destroy call

Adding missing bpf_iter_vma_offset__destroy call and using in-skeletin
link pointer so we don't need extra bpf_link__destroy call.

Fixes: b3e1331e ("selftests/bpf: Test parameterized task BPF iterators.")
Cc: Kui-Feng Lee <kuifeng@fb.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20221006083106.117987-1-jolsa@kernel.org


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>