selftests/bpf: Modify linked_list tests to work with macro-ified inserts

selftests/bpf: Modify linked_list tests to work with macro-ified inserts

The linked_list tests use macros and function pointers to reduce code
duplication. Earlier in the series, bpf_list_push_{front,back} were
modified to be macros, expanding to invoke actual kfuncs
bpf_list_push_{front,back}_impl. Due to this change, a code snippet
like:

  void (*p)(void *, void *) = (void *)&bpf_list_##op;
  p(hexpr, nexpr);

meant to do bpf_list_push_{front,back}(hexpr, nexpr), will no longer
work as it's no longer valid to do &bpf_list_push_{front,back} since
they're no longer functions.

This patch fixes issues of this type, along with two other minor changes
- one improvement and one fix - both related to the node argument to
list_push_{front,back}.

  * The fix: migration of list_push tests away from (void *, void *)
    func ptr uncovered that some tests were incorrectly passing pointer
    to node, not pointer to struct bpf_list_node within the node. This
    patch fixes such issues (CHECK(..., f) -> CHECK(..., &f->node))

  * The improvement: In linked_list tests, the struct foo type has two
    list_node fields: node and node2, at byte offsets 0 and 40 within
    the struct, respectively. Currently node is used in ~all tests
    involving struct foo and lists. The verifier needs to do some work
    to account for the offset of bpf_list_node within the node type, so
    using node2 instead of node exercises that logic more in the tests.
    This patch migrates linked_list tests to use node2 instead of node.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230415201811.343116-7-davemarchevsky@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>