Use list_heads properly. In a few places we were using them incorrectly:

Use list_heads properly.  In a few places we were using them incorrectly:

BAD:  list_add(&list_item, list_head.prev);  Overwrites the adjacent field.
GOOD: list_add_tail(&list_item, list_head);

While the following usage is technically correct, it does encourage people
to use the above (incorrect) usage pattern, so it should be avoided:

BAD:  list_add(&list_item, list_head.next);
GOOD: list_add_tail(&list_item, &list_head);

Just FYI - list_add() is like a queue, while list_add_tail() is like a stack,
when iterating over items via list_for_each().