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

tools/testing/selftests/bpf/prog_tests/bpf_iter.c

@@ -1498,7 +1498,6 @@ static noinline int trigger_func(int arg)
 static void test_task_vma_offset_common(struct bpf_iter_attach_opts *opts, bool one_proc)
 {
 	struct bpf_iter_vma_offset *skel;
-	struct bpf_link *link;
 	char buf[16] = {};
 	int iter_fd, len;
 	int pgsz, shift;
@@ -1513,11 +1512,11 @@ static void test_task_vma_offset_common(struct bpf_iter_attach_opts *opts, bool
 		;
 	skel->bss->page_shift = shift;
 
-	link = bpf_program__attach_iter(skel->progs.get_vma_offset, opts);
-	if (!ASSERT_OK_PTR(link, "attach_iter"))
-		return;
+	skel->links.get_vma_offset = bpf_program__attach_iter(skel->progs.get_vma_offset, opts);
+	if (!ASSERT_OK_PTR(skel->links.get_vma_offset, "attach_iter"))
+		goto exit;
 
-	iter_fd = bpf_iter_create(bpf_link__fd(link));
+	iter_fd = bpf_iter_create(bpf_link__fd(skel->links.get_vma_offset));
 	if (!ASSERT_GT(iter_fd, 0, "create_iter"))
 		goto exit;
 
@@ -1535,7 +1534,7 @@ static void test_task_vma_offset_common(struct bpf_iter_attach_opts *opts, bool
 	close(iter_fd);
 
 exit:
-	bpf_link__destroy(link);
+	bpf_iter_vma_offset__destroy(skel);
 }
 
 static void test_task_vma_offset(void)