selftests/bpf: Add missing bpf_iter_vma_offset__destroy call

selftests/bpf: Add missing bpf_iter_vma_offset__destroy call

[ Upstream commit 1d2d941bc140b34587b4c889699fb0f89d29937f ]

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: b3e1331eb925 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>